The trash mobs actually give a good chunk of XP per kill, even scaling down to a lower level dungeon you'll still get a good influx of XP from the monsters themselves. The end reward isn't great, but the XP per kill seems to compensate for that.

My problem is the dungeons becoming not viable financially towards the end of the game, but that's a different matter!

Like others have said, I think the dungeons are as designed and pretty well balanced (in terms of XP. Gil wise? Total disaster.)

It's the FATEs that are unbalanced and problematic. Virtually no work for rewards that aren't remotely proportionate to the effort any one player is putting into it. If you solo/duo a fate you'll walk away with the same XP/Gil/Seals as if you'd done it with thirty people. That's just weird.