Level scaling isn't necessary for a nonlinear game. Plenty of games have nonlinear areas and no level scaling because they have relatively flat progression curves. D4 only 'needs' level scaling because it also 'needs' to be Cookie Clicker and Lootbox Simulator with an ARPG wrapped around it, and neither of those systems work with flat progression curves.
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.
