As for HD redos, for movies this isn't an issue: they were all made on 35mm (or 70mm) film. Yeah, for some old 80s-90s show that was tragically shot on videotape, it's going to look like shit no matter what, so you're just stuck with that unfortunately.
Doesn't matter when they remaster it to be way too dark or screw up with colours and tone mapping to give it a "more modern" tint.
Some are also re-cut from original aspect ratios (whether 4:3 or CinemaScope) to be equal or closer to 16:9.
e.g The Wire on DVD is 4:3 (as originally shot) and has been re-framed to 16:9 on BD. They did a a seriously good job at it but it really changes the impact and/or mood of some key scenes. And that's when it goes well.
On the other end of the spectrum I've seen 2.55:1 movies butchered into some lazy [anything-between 1.77 and 2]:1 crop + autopan conversion "remaster". Those are 1080p alright and not upscales but are a totally shoddy cash-grab job.
https://www.digiraw.com/DVD-4K-Bluray-ripping-service/4K-UHD...
I don't think anyone upscaled a dvd recently for release on BR though, was mainly an issue right around when BR came out in the rush to get content out. Same with UHDs, today it shouldn't be as much of an issue.
The movie studios still manage to mess it up, changing the colors/contrast and using terrible AI de-noising that wipes out actual details
https://youtu.be/oZWNGq70Oyo
And honestly a ton of why dvds look so terrible is people forgot about deinterlacing. Toggle it on and things look a lot better.