I mean I get it in part. If you go at the angle that “combat should be a tax, not a desired outcome”, then 4es combat focus isn’t your thing. If you liked vance things for worldbuilding, then 4es abstracted power system can also be a turn off.
But 5e has all the things grognards theoretically hate: forgiving combat (requiring quad taps to be put down),at-will, encounter and daily powers, innate healing, still has a combat focus, alternate maths, and while it lacks 4es power stuff, has 3es munchkiny multiclassing nonsense. Which theoretically 0D&D fans hate.
If you like what 3E and/or 4E is doing, it does it pretty well. If you dislike what they are doing, they are (relatively) hard to change. 5E's defaults suck, but it is much easier to change into something playable for a given style--even easier than the early editions in some ways.
Now, I can't say about how the supplements might have botched things, because I got off that train early. But the initial 5E didn't have a lot of multi-classing nonsense, because in practice it gets used very little. If anything, multi-classing messes with the munchkins for the most part. Feats are easy to ignore. Heck, I grudgingly allowed then in one campaign, and had a total of 2 feats taken over about 30+ opportunities. Toss in some DMG options to make combat deadlier and then use the Kobold Press monster books instead, and you get something deadly enough for some of us. As deadly as 3E was out of the box, maybe a little more so with exhaustion. (Bit easier to lose a character in early 3E, a bit easier to have a TPK in early 5E with the options turned on.)
Now, in practice, what I want on deadliness is somewhere between a dialed in 5E and the early stuff. Not everyone that enjoyed early D&D thought that characters dropping like flies was always its best feature. (Great in some games, something to work around in others.) I've incorporated some BEMCI/RC features and some 5E features (dialed way back) to get a balance I actually like better in my own design. So I'd say what you are missing is that not all of us that generally prefer old school liked every single detail of old school design and implementation. Sure, there are people here that like that, too, but not all of us.
There is a dichotomy there that comes up a lot in design, I think, and not just limited to game design: If you want to appeal to a diverse group, it's about how well you widget does what it sets out to do, how many people want to do that, but also how easy it is to repurpose the widget for something nearby. Early D&D being fairly to repurpose in many ways, how it got played varied accordingly.