The arguments about how small guilds had an advantage at the higher levels (because the small guild bonus countered decay) were always red herrings, since the vast majority of small guilds by definition had too few members to muscle their way to those levels in the first place. I showed that virtually all of the small guilds that were able to get to the higher levels were able to do so only because their members on average were simply several more times more productive than large guilds of similar levels. Large guilds could get good ship amenities like +2 stat and 30 resist shrines just by size alone, whereas for small guilds it was either be very productive or do without. So those small guilds that were active had an incentive to work extra hard at renown (taking heroic deeds over major mana pots, etc.), and they simply carried that mentality to the higher levels; their ability to continue gaining levels in the 70s and 80s stemmed from this guild atmosphere, even though large guilds kept saying small guilds advanced because the system rewarded small sizes (and hence why the system needed to change, i.e. reward large guilds even more), which had the logic completely backwards.
The sad commentary on this is that large guilds were busy saying that they had no choice but to boot people due to decay (i.e. they don't see encouraging members to be more active to be a reasonable option for them, and it's the system's fault they were held back in levels), yet when pointed out that they were far and away years ahead of small guilds in level already (i.e. even before the change), they said small guilds were simply lazy and wanting things for free without any effort, and that small guilds just need to work harder at it (i.e. it's the small guilds' fault for not encouraging members to be more active). Turbine or anyone else can look through the linked thread and see these arguments in abundance. Yet Turbine bought this argument without analyzing whether or not what they were saying was correct (i.e. how the system actually worked mathematically), nor considering the hypocritical and self-serving position of wanting rewards for themselves and yet denying it for others, even though they were a distinct minority of players.
Making a 6-account guild equivalent to a 24-account guild in earning power probably sounded quite reasonable if Turbine only considered guilds up to 50 accounts (since they would gain renown half as fast as the largest guilds considered). Similarly, a 16-account guild, equivalent to 40 accounts in earning power, gaining levels 80% as fast as a 50-account guild probably sounded reasonable.
But the system heavily rewarded large guilds for increasing their size beyond 50 accounts, and Turbine probably didn't expect the rise of 600+ account guilds that would go around arguing how Turbine favored small guilds with the system, or shedding crocodile tears at having to boot their more casual players (rather than, you know, encouraging them to play, which is what the large guilds expect from the smaller guilds). They probably didn't expect 100-account guilds going around saying the small guild bonus -- even a pittance as it was -- was a "pseudoexploit" and claiming that the only reason anyone would want to be in a small guild was to optimize the guild size for guild levels, even though that's a horrible renown strategy, and an insult to the vast majority of players who prefer small guilds for social reasons. Thus the focus on renown decay, even though the vast majority of guilds is hindered by the small renown gain.
I would venture to guess (though don't have evidence) that the developer(s) that changed the system last October probably wasn't the same one(s) that implemented the renown system in the first place, and didn't consider the past internal and forum discussions about the system when it was being put into place, especially regarding different size guilds, nor the far-reaching effects of changing a system that already heavily benefited one small segment of players, into a system that benefited the same segment of players even more.
As Turbine themselves said, even before the renown system was implemented, the average guild size was 12 accounts. In other words, when the guild system was purely social in nature (i.e. before a points system was put into place that favored one size guild over another and distorted the average guild size), players naturally preferred smaller guilds. This makes sense, since many players say they play the game because of the people they meet and the friends they make, and smaller guilds is a way for people to self-organize around common interests, playtimes, etc., and build individual relationships. As I've mentioned before, I could log on in a small guild and half the players online will say hi to me; I can log on in a large guild and nobody will pay any attention to me for the entire day. It's the close-knit environment that brings people together, and encourages each other to continue logging in.
This doesn't mean that everyone wanted to be in a small guild, just that the majority of players demonstrably did. Players should self-organize around whatever size guild they want, and the system should encourage all size guilds relatively equally. Yet Turbine or anyone else can read through the linked thread and see how people from large guilds disparaged anybody from small guilds, going as far as to call them "not really guilds" and saying that people from small guilds are just staying small to take advantage of the size bonus for guild levels (while ignoring the effective size bonus that large guilds by definition get by having many more times the number of players that can gain renown -- and that staying small actually makes it harder to level). And this even though pretty much all renown systems discussed in the thread, whether proposed by me or other players, still demonstrably give an advantage to large guilds -- just not as enormous as it stands under the current system.
By having the system encourage everyone to be in large guilds, Turbine is moving players away from the community-building aspect of guilds, that encourages players to get to know one another individually and spurs each other to want to log in. For the casual player, the guild choice is fairly stark: either join a large guild where members will ignore you once you're in (except to boot you if you don't log on for a few weeks, to make way for others that will help them get renown) and you're on your own to learn the game, or join a small guild where you will get individual attention but will likely have to do without good guild ship buffs, making the initial content that you experience much more challenging (since ship buffs make the most difference at the earlier levels). This was already the choice presented to casual players several months into the renown system several years ago; the choice is simply in sharper relief now with the change last October.
Of course, Turbine can change this at any time if they wanted to. They've shown that they can change the guild size bonus on the fly, and from a coding perspective, just like with renown decay, it's a simple change to make and doesn't need additional code, since it's just changing a few variables (namely, the table lookup relating # accounts -> # renown bonus). It's only a matter of whether or not Turbine wants to. And given how much large guilds protested the renown size bonus when they were already above "only" 98% of everybody else, claiming it was putting them at a disadvantage (despite being demonstrably false), I can only imagine how much forum fury will be unleashed if Turbine really unlocks the higher levels for casual small guilds. After all, the current system only allows the most active small guilds and all large guilds, casual or powergamer, to reach the higher levels, so large casual guilds have a vested interest in seeing that they retain their monopoly on the higher levels for themselves and not other casual (i.e. small) guilds, since it would dilute their recruiting pool.
After all, large guilds loved the change, since it meant that they get more out of the miniscule effort put into pumping then dumping random faceless players, and with a higher guild level it's easier to attract more players into this scheme. By their own admission, large guilds consist of a core of active players -- say roughly 10-20% of the guild -- and then mostly more occasional players. So it's great for the core to clamor for changes where they can get more benefit from this strategy (by having a larger pool of players wanting to join) and less drawbacks (by decay not depending how many players -- so any additional player is pure gain and never loss). Because Turbine instituted the change to renown decay, now there is even less (not more) incentive for large guilds to engage their members that aren't part of the core group, because of the time and effort involved in actually encouraging the non-core members to play (i.e. get them interested in the game). It's much easier to just sort the guild by last login, remove the X characters at the bottom, to make way for newcomers to the game that are lured by high guild levels. The core players from those large guilds that post frequently on the forums think it's completely awesome that the system rewards their strategy, but it's actually a disservice to the rest of the guild (who end up not being engaged by Turbine's social system, i.e. guilds), as well as to the vast majority of other guilds that don't use such strategies to level up, and aren't rewarded for their individual interactions with players as a result.
Because of this, and given how vehemently the large guilds lobbied for the change to renown decay in the first place, Turbine understandably does not really want to openly discuss the issue in depth. Posting their thoughts essentially means they need to take a position, which leads to a lot of angst from anyone who disagrees with their position, regardless of how well-thought-out it is or how much evidence they have for it. Even though the system provably heavily rewarded large guilds, large guilds claimed they were being unfairly harmed by the system anyway, until Turbine changed it. So it's much easier for Turbine to say nothing now and let people feel whatever they want rather than alienate a vocal minority of the playerbase and cause forum drama, despite the vast majority of the players being hurt by the current system, and despite the current system being a bad strategy for long-term retention of players, especially newcomers to the game. That's Turbine's choice to make.