Current system without decay: Leveling system
Unless there's a reason for players to continue buying renown potions after their guilds have reached max level, changing the system to no longer include decay is akin to Turbine shooting themselves in the foot. There's a benefit to players continually pumping money into the system. More profits = paychecks for employees.
I find it more realistic to believe that the total number of players in guild sizes 10+ would grossly outnumber the total number of players in guild sizes 10 or less.Small guilds are actually by far the most guilds out there, if I remember Vanshilar's count right.
One mega large guild is likely to encompass a large total of small guilds. We have to take into account the sum total of players affected over the number of groups affected.