Having run some hardware for about 20 years (recently deceased), the problem that eventually happens is that newer OSes drop support for old hardware. If you hit some weird bug on your setup on a new OS release, there won't be anyone to help you fix it[1]. So then you're stuck on an old OS. In time, that means you can't run the latest userland software either, which relies on more modern OS features (eg: your Firefox will get increasingly out-of-date). That means the set of things you can do will eventually narrow and narrow.

If you're only running programs that you have full control of, and can compile/fix locally, or where receiving security fixes &etc. don't matter, then you're good. But things are a bit more interconnected, these days.

I do still enjoy running my hardware into the ground rather than tossing out perfectly good components every few years though (:

[1] In my case, the boot loader stopped working for my hardware on FreeBSD 11.4