It's literally an endless grind because enemies scale with you as you level up, so all mobs feel the same and there's no sense of progression.

This, part of building your character is the power fantasy you get when you go to weaker zones and absolutely obliterate everything.

With the current system, you actually end up getting weaker as you level up

But why should I do that? I don't get that argument. I've never went back to a low level zone in any game just for the purpose of "obliterating everything". If there is a quest I missed, sure. But what's the point doing that?

Diablo 4 makes you go back and forth across zones, GTA-style. Not only for sidequests, but also the main storyline. D2 solved this by artificially constraining you to one region per act in a linear fashion.

In D4 when you pass through or go back to a zone you've been in, and the monsters are tougher, it feels awful.

AFAIK D4 does that in some capacity. There are zones that scale from 50-100 for example and if you are below 50 you'll have a bad time. It's correct that once you visit a former zone the monster scale with your level but as soon as you have the mount you don't have to put up with them. And because of the game design (a lot of dungeons, world bosses, legion events etc.) it's a welcome thing that they happen all over the zones and are not restricted to some high level area.

That has not been my experience. Yes, at certain moments, I am pressed to optimize. However, I'm currently wrecking WT3 with my necro. I know it won't last forever as I scale up, but that's fine with me, otherwise I'd have nothing to look forward to.

idk, i feel like items are how you progress - My character at 48 with upgraded legendaries is a badass compared to 28.