[kaoD]

Sorry I don't see what you mean with flat progression curve or how it solves anything here.

If I'm level 70 and I go to a level 15 area, how would that solve that I'm just going to cut through the area like butter?

[YurgenJurgensen]

By not making level 70s cut through level 15 enemies like butter. If your level 70 is only 50% stronger than a level 15, if the enemies were challenging for a level 15, they won't be trivial for the level 70.