Despite VoN 3 LFMs outnumbering all other runs of the same level combined at times, other quests were still being run. Most people get kind of burnt out running the same quest too many times. I know that, for me, that limit is 3-4 runs. After that, I don't care what the XP/min is still, I'm either running something else or logging off. That said, there are quests that no one can get a group for, because the XP is just so awful that it's really not worth running the quest more than once or twice. Now, if you happen to really enjoy a quest, or the loot therein is good enough XP/min may not matter, but both of those draws will lose their power to populate the LFMs long before XP will.
That your response is to be concerned that a level 26 quest that offers 60,000 XP for 20 minutes is excessive just shows that you're looking at this picture in the wrong way. If we can get 20,000 XP (low ball estimate for some Vale quests) for 20 minutes in a level 16 quest, when we need 165,000 XP total to gain level 17, why is a level 26 quest that takes the same amount of time to run, and yields 60,000 XP a problem when the total XP required to get from 26 to 27 is 1,050,000 XP?
For comparison, at level 16 20,000 accounts for about 12% of a level going from 16-17, while at level 26 60,000 accounts for about 6% of a level going from 26 to 27.
So, DDO is based (loosely, and loosening more) on D&D. In the pen and paper game, the XP required to get from one level to the next increases with each level gained (1,000, 3,000, 6,000, 10,000, etc...), which is similar to the curve we see in DDO, but in D&D the number of encounters required to gain the next level stays fairly static from one level to the next. I believe the 3.5 Dungeon Master's Guide pegs it at either 11 or 13 at-level encounters per level. Now, those figures can go up and down a bit depending on what your group faces (it may be only a handful of encounters if they are all 3+ levels over the party's level, for example, or may be more than 13 if they face a bunch of softball encounters). In DDO, we look at quests/level and XP/quest rather than encounters/level or XP/encounter because the game is set-up differently, largely as a factor of time: it takes about as long to run a DDO dungeon as it does to run a combat encounter in D&D. Additionally, DDO is necessarily looking to draw out the leveling process in a way that D&D isn't looking to do (on-line gaming and all that).
In DDO, XP/level curves upwards significantly, while XP/quest is a much flatter line, thereby requiring more completions of quests per level. This is a problem. On its own, it wouldn't necessarily be an issue, except for two things: 1st, there is less variety of content at the highest levels of the game than we have at lower levels, which necessarily means much more repetition of content, which in turn leaves players feeling like they're grinding XP rather than leveling to enjoy the game. 2nd, we aren't tied to only running quests at our level. Now, that may seem like it contradicts the first point, but it doesn't really, not when we're looking at how much XP quests should be granting. If a quest at level 23 is granting 3,000 XP/min, and a quest at level 28 is granting 500 XP/min (and the level 23 quest is going to be easier than the level 28, by virtue of being a lower level), players are going to run that level 23 quest more often. In heroic content, this is a little bit less of a problem (just a little), because XP penalties kick in once you're two levels above the quest's level, so you can milk a particular quest for only so long, but with the wider range on epic content, there is little stopping us from just running the easier, faster content that is also offering better rewards.
Look at the leveling patterns for some players (I won't try to quantify how many vs. the rest of the game's population): milk Litany of the Dead (one of the best XP/min quests in the game) from level 16 or 17 to cap, or until you need to pick up a level, and then hit some Vale quests to get the rest of the way, skipping most of the content available for levels 18 and 19 (and 20). Why is that? Because the quests in that range are significantly tougher than Litany, and not only offer worse XP/min than Litany, but offer worse XP/min than several quests of lower level.
Really, your starting baseline for assessing quest XP should be to start with the total number of quests you have for a given level at the time you're making the quests, and then divide up the total amount of XP required to advance to the next level and divide it by that number of quests, maybe lessen the shares by a bit to allow for some repeats, or a quest that offers better than standard XP. There are currently six level 26 quests in the game. Even assuming they should be run twice each, they should be netting us about 87,000 XP per completion each. 60,000 XP for the completion of a level 26 quest is low, but not so low that it's totally inappropriate. The problem then stems from all the rest of the level 26 quests having significantly worse XP. The situation for level 27 is worse, with only 5 quests, and for level 28 is awful, with only a single quest.
Asking us to repeat each quest 3 times or more for a given level is part of the reason so many people have found the game to be excessively grindy at higher levels.
You can do better than this.