1 game day = 365 actual days is not 100% accurate for all things in game, is it? Lots of things running in game have their own time lines that don't line up to 1=365. Like some crops growth and equipment operating hours. 5 years old with only 30 operating hours should mitigate some depreciation for age. Unless Giants wants to multiply operating hours at something like x14 so it fits better with the time scale. Having actual operating hours is good for the player and is a fun stat to track so I hope they don't change it.
Farming on PS4
I have dirt "off", but the equipment needs repair anyway, especially implements. You could edit the vehicle xml file and prolong the lifespan of things, but it's a lot of work and if you don't save a copy of the files the next patch will probably reset the values, as the 1.2.1 did for me.
You don't have to sell them. Keep those smaller tractors around. They still work great for fertilizing and lots of other things that you don't really need a big tractor for.
Playing on PS4