But your point about equipment lasting many years is certainly true. We have an International tractor that is a mid-fifties model and, although it has had several engine or transmission rebuilds, it runs hard and strong. That being said, it is no longer the best piece of equipment we have for many tasks. It works around the edges in a stand-in situation, or works in the hay field.
There are also large farms that lease all new equipment every 2-3 years. In one case with a neighbor of ours, he leases 20-30 of the latest and greatest tractors every couple of seasons (worth $200k each). That doesn't work for us, but it is certainly done around here.