I have a theory, but am yet to test it.

Surely around lvl 30-40, where rudimentary skills such as your weapons and armor/pref magics/pref crafting are maxing out, you're gonna be the don. But what about once those skills can't be improved further? The character will still be increasing in level through minor skills but your 1-handed for example will forever be the same and, assuming one has full legendary daedric/dragon, that's not gonna get any better either, thus causing difficulty to increase.

Discuss.