Actually, I don't mind this. Hell, even if lower level dungeons give you fewer tomes.

Why?

Because there's a whole class of new player out there who're still working their way through the story. This at least gives some incentive/reward for higher level players to 'step back' and help out newcomers or their friends in getting through earlier dungeons in the game. One of the biggest issues often stated with dungeons, especially if you're a DPS, is huge queue times and incompetent groups; the answer to these is always "make friends and queue up with them", but even if you ask your FC who might all be on at a given moment to help you with a lower-level dungeon, chances are they're all not interested because they're all off doing more relevant content to them personally.