All things equal, I'd expect my 38 year old Massey to still be plugging away in another 38 years and the 2 new Deere 6430's I just recently took delivery on will be parked in a fencerow in 25 years. Once major components start failing, they'll just be too impractical to put back on line. (read; TOO EXPENSIVE) HArd to say.....Check back with me in 2034 and I'll give you a better answer.
I hope you won't be parking them 6430's
Taken care of properly those will be worth as much then as now
We have been running a 7210 for about 13 years with no issues yet and now a 7220 and a 7420