IDK, a tractor is like a wife, sort of. You find one you like, the two of you work together to make a better life. Just because she gets old is no reason to kicker her to the curb. Now if she died or ran off, thant certainly check out what is available....
Though on the financial side, a new tractor may be cheaper to own. You can also not take it with you. When my dad was getting up there, I told him he should fill the house with topless 20 year old French girls, and die with a single nickle. He wanted to leave his kids with something. What resulted was half being eaten up in lawsuits. You worked for it, you enjoy it.