My Sins are variations of each other.
Yeah, these are both mine too. If I want to make something easier for myself I can very easily turn the difficulty down - but I'd like that to be my choice.
I think that a degree of monster levelling is ok, but only within certain constraints:
1) That it only results in a certain change in the base stats - say, 30-40% either side, so that the enemy remains roughly the same strength.
2)That you provide a degree of variation at all levels. So if you face 10 wolves in total at level 1, you'd expect 6 of them to be weak, 3 to be medium, and 1 to be strong.
If you face 10 again at level 10 you'd expect 2 weak, 5 medium, 3 strong, with a somewhat diminished, but not insignificant challenge.
But by the time you hit level 30 you'd be fighting almost all strong, but the fight would nonetheless be completely trivial.
3)Thirdly, that you mix up enemy times as characters progress. I'm not saying that low level creatures should be completely eliminated at higher levels, but they should be supplemented with new types, or greater numbers, or more challenging tactics.
For all of these, I think that the Morrowind midgame is a great example of how to do it. They managed to incorporate it into the story by having disease spreading during the story, producing variants of animals which hit harder and attacked more ferociously. You could also contract the diseases.
The way that the enemy spawning worked was to use 'levelled lists'. So any enemy you faced would have maybe 20 different spawn options. The options would be linked to level so you'd usually get an enemy roughly commensurate with your level, but it was still RNG'd for variation - so one time you'd get something pretty challenging, another time something less so. Some spawns would only start at a higher level so as to make some areas way too tough for low level characters.
Additionally, there were certain spawns which included multiple creatures. So you'd face a rat at L1, a wolf at L5, and an entire pack at L20.
To supplement the levelled lists, there were also fixed spawns, so you'd still get utterly mulched if you went somewhere dangerous at low levels. And it then felt somewhat satisfying to return to those areas later.