I've gotten the impression that what Deere has done doesn't keep you from replacing parts; it forces you to pay for someone to go out and authorize the replacement for the tractor. I expect that the idea is the cost of doing so, and the inconvenience, is intended to make it so it's not worth doing any sort of third party repair. And while I expect that the "authorized repair is safer" is part of how they're trying to legally sell it, it's ultimately just about the money.