I don't race.
For recreational rides I've never felt handicapped on my 1989 KHS.
Of course I've never owned or used a "modern" mountain bike. I'm pretty sure the newest I've owned was an early 1990's Nishiki.
Considering the fact that I don't race, ride alone, and at my age will never come close to approaching the performance limits of the bike I have now, never mind a new mountain bike, I don't see any reason to "upgrade" to something new.
Heck, for that matter, it would be a waste for me to get my "grail bike" that I've wanted since 1968 or 1969. (Cinelli Super Corsa with full Campy Super Record) To be honest, I doubt my abilities ever warranted that bike.