Catch 22 really. If you add rewards then you
a) add power creep, further causing problems with not enough difficulty or too fast to the non-existent "end game"
and/or
b) add perceived requirement to run higher difficulty for the favor/xp, making people complain about the increased difficulty
Until someone can suggest rewards that don't do one of the above, its quite the conundrum. I made a suggestion maybe a week ago in some thread (could have been this one even) that they add a sort of prestige system: add a new difficulty harder than elite, maybe every completion grant a stacking semi-permanent debuff (say -1mp and -1sp) but granting you a stacking hash mark or star indicating like an ace status. You get a permanent * next to your name for the highest level of hashmarks/5 your character has ever achieved. This adds challenge to those lacking it, adds goals, adds some kind of incentive, but doesn't add any power creep.
But it's just an idea, we need some other ideas of how to address the conundrum above.