define 'longevity'... Sort of like the original axe I got from my grandpa... 2 different axe heads, 4 handles, but its the same axe.
Anyway, by the time you have hit 50k (or whatever high mileage), you will have touched pretty much every part of the sled, multiple times. Any part that wears out will have have most likely worn out and been replaced. Not to mention, maintenance and riding style are far more important than the number on the odometer.
Anyway though, as others have mentioned, yamaha is known for reliability. Even owners of other brands will admit that one!
1) 2004 Yamaha Warrior
2) 2006 Yamaha Nytro