If the price is right And it is something to get started on... sure.
For hardtails, the difference will not be as big as for FS, but even for the latter, if the price is right a 2010 FS will still take you far.
Yes, geo has changed a lot in the pst 5 years. But here is the thing: I am riding a frame from 2012. Turner. 5-Spot which is one of the last of the high end 26ers. In the time since I got it, most of the guys I have been riding with for the past 20 year have gotten new bikes with updated geo. They all like it. You know how much difference this has made in the pecking order of climbs, decents, and who can clear what stuff? Practically zero. I am sure a stopwatch or some careful metrics would show a difference (particularly on downhills) but no one is riding stuff they could not before.
Would I spend a lot on a high end 10 year old bike? No. Why? Because the standards are all phased out dead ends. And money you put into most parts are going to be useless to you moving forward. Selection of high end 26” tires are getting thin. No new high end forks will work. Axle standards are all different. Even fitting new drivetrain options gets wonky. I love riding my 5 Spot, and just did a full rebuilt of all the suspension (pivot bearings, shock/fork rebuilds), wheel bearings, etc, and it rides like a new bike. Yet even for this like-new high end bike, I would not pay even $1K for it if I needed a new bike today. It locks you into old geo and old standards.
Last edited by Kapusta; 04-13-21 at 07:54 AM.