My brother-in-law has a large operation, a bit larger than 10,000.  I once asked him why they didn't use those big rubber tracked tractors.  He said they had tested the Cat version which needed a bit wider implements and than they used,  plus they rode so darn rough, he thought they'd jiggle his operators to death.    Going by memory here, I also asked him how long they run there tractors before retirement and his answer was "about 10,000 hours".