If you don't care about retro purity, you can ride them much harder than originally. I've had my 1993 Stumpjumper since new. Like most bikes that are really ridden, it was modified/personalized as soon as it came home. Since it's been my main bike for most of the time since then, it's sporadically had parts replaced due to wear, but also improve fit and performance. Few original parts remain on the bike. It's a vastly better performing bike now than it was in the '90s because of the changes.
In the '90s, it was really sketchy on steep downhills due mainly to the low bar height and width and long stem. Even set at the maximum height limit line on the quill stem, the bars were around 9cm below the saddle, which, with my slightly negative ape index (shorter arms than average), had me riding with a road bike flat back fit. I had also put on 56cm wide Zoom Brahma bars. They were decent for easy trails and road coming from a road bike back then, but I didn't realize the negative effect they had on handling off road, particularly in downhills and carving corners. Now, with much wider bars (74cm, 17degree backsweep to keep my wrists happy), a higher position about level with the saddle (partly due to longer fork), which gives me a much more upright position but still good for cross country/steep climbing and 2cm shorter stem (but still considered long at 11cm), it's gone from sketchy to fun on downhills. Of course, I'm talking about trails good for cross country bikes, which is 98% of the trails in my area anyway, not really gnarly stuff.
Other key changes that improve on the '90s setup of the bike:
- Retro bikes had long brake levers, 2.5 finger on my bike. With a one-finger lever setup, more fingers stay wrapped around the grips and you have much better confidence on the bars when it's fast and rough. With well dialed in V and canti brake (and Kool Stop salmon pads), braking power is good with one finger braking on my bike.
- Specialized Future Shock 46mm fork (Rockshox Mag 21 internals) to 1997 Marzocchi Z2 Bomber (65mm). Though it's moving from retro fork to retro fork, and the Z2 is much heavier and shorter in travel than current XC forks, they hold up well in performance today, unlike the original forks, which I couldn't get parts for anyway.
- The original 3x7 gearing wasn't low enough (26x30 low gear) for the climbing I like to do. I've had 1x on a newer bike, and that only confirmed that I like 2x best. I'm now on 2x9 with a 24x34 low gear, and I can easily do tech/steep climbs that I couldn't do back then.
- Mountain bikes had longer cranks than road bikes back then, probably because they didn't fit low enough low gears. Ditching the 175mm cranks for my favored 170mms improved ergonomics a bunch for my leg length (no longer pedaling squares). The ground clearance for pedaling over roots also improved (especially combined with longer fork and larger tires).
- 1.95 to 2.2 tires are a big improvement off road. I like fast rolling tires, and the Continental Race King RaceSports are perfect for where I ride.
- Retro saddles didn't have perineum relief, which I could get away with, but a modern saddle is comfortable for longer.
I had a 2015 Stumpjumper Elite 29er hardtail (bought used in 2017) for 3 years until it got stolen out of my house (burglars left the '93 Stumpjumper). It had carbon Roval wheels, a high end Rockshox SID Brain 80mm fork, carbon XX1 cranks, carbon bars, Shimano XT M8000 1x11 and brakes. As a cross country hardtail, it is a good comparison to the kind of bike the '93 bike is (rather than comparing it to a bike park bike or long travel hardtail). It's not that different than the hardtails a World Cup cross country racer might still occasionally choose to use, but also not that different to the '93 bike. The '15 bike was 2.4lbs lighter than the '93 is currently (though I could probably get it down to the same weight), but I think they'd clock similar times on a race course because I have much faster tires on the '93 than I had on the '15. I know racers have done controlled, timed comparisons of different wheel sizes, and the differences between 26" and 29" (and 27.5") are actually kind of hard to tease out, and depend on the course.
After the '15 bike was stolen, I put more effort into dialing in the '93 bike just right, and it wasn't long before I realized that the performance differences were essentially zero for the kind of trails I like to ride. Since I also value keeping durable goods going instead of throwing things away every few years, and have a long attachment to the bike being the original owner, I've decided not to replace the stolen '15 with a modern bike. I also like the idea of owning just one bike (not counting the road bike I left at my parent's place in the '90s).
note, doesn't look broken (refer to other discussion on modern geometry) 
