I got so close to re-buying a couple 3.5 books last year. So glad I didn't as after the year long slog of running a 3.5 hexcrawl, I was asked myself why I didn't just use Pathfinder 1e. I feel I finally killed the idea that 3.5 had specific things I preferred over Pathfinder. It doesn't. My love for the game was purely aesthetic or sense-memory related. And while that can be valid if that aesthetic inspires me as the GM, I've now been able to lay that to rest.
The only Nostalgia books I am going to keep are my Bubblegum Crisis books as that was the first game system I ever ran properly. Most games before that living in the childhood realm of "Chuck dice, pretend to reference book, make something up." I had been doing before.