I think the thing that wears out tractors now a days, it's either livestock where it runs every day, or it just gets passed on for better technology even though it's low houred.  There's a wave of equipment manufactured early-mid 00's that are plenty capable, but weren't autotrac ready.  A guy could spend a few grand to update it, or put the money into a newer machine with it already integrated.  Also, look back at the 55-60 series deeres.  A guy could keep on running one just fine, but look at the hydraulic capacity and the size of the cab on that 8400 when it came out.  Time to trade.  Seems like today, it's the high horse that I can now trade up and pull a bigger implement, or pick up a mile and hour or two with the one I got.  Time to trade.  Next up could be an automas tractor, or something completely different.