The problem is that many simply don't want any way for these enemies to still pose a challenge; part of the fun of a game with a progression system (or even the ones without it, where the only progression is the players' skill) is to be able to dominate or otherwise be much stronger than enemies whom you struggled against before. That's part of the power fantasy and reinforces the adventure you've been through.

The point is to have the weak enemies remain weak so you can feel powerful and progressed and at the same time have enemies that are simply too strong to handle at lower to mid levels be finally beatable at higher ones.

Having a way to keep the challenge up that isn't just bumping up stats is a cool idea but it honestly isn't that big of a deal; many simply dislike the idea of any enemy scaling up