Earlier quoted context omitted.
I don't think they shot Seinfeld in 35mm to be future proof. It was the best way to film at the time.
Friends reruns also got the HDTV rescans. In some shows you can see things that weren't supposed to be on-camera in the 4:3 format. It's just a matter of which shows are worth the cost and time to do it.
I can't find it now, but I believe they even redid some of the early CG effects by just rerendering the old models with modern software. It's worth preserving that kind of stuff for future generations. One of my favorite video games was rereleased over a decade later with updated graphics. Sadly they had to recreate everything from scratch because the original files were lost. And the original models were very high detail. They just converted to 2d and down sampled a ton because of the limitations of PCs at the time.