The issue is the Sacred Cow of "Plate armor is AC* 17-19". It's too low. A adventuring suit of Plate Armor should be AC 20.
If you go the bounded accuracy route, even the weakest monster monster has a 14 in their attack score and proficiency in their weapon. That still lets them hit a 20 with a high roll. And considering that 5e doesnt give you the best armor at level 1, it's not even that bad.
If normal armor went from 11 to 20, we'd have just that much more room for missing rules like Touch AC, Partial Armor, Banded armor, Brigandine, and Multiple Shield types.
Cross fingers for 6e.