30 years ago, John Deere had that Network of service technicians and ready on demand parts. It was probably slightly smaller, then, and many of the farmers (both small independent and large corporate farmers) had more of a stock and more knowledge of how to repair their tools, but not fundamentally different. Harvest being crunch time was not brought about by the computer revolution. Harvest being crunch time was not brought about by just in time logistics. Both have allowed John Deere to do more with less by understanding where it's stock of inventory is better, rather than having to have a staff of quartermasters running those books. That efficiency is not because of their DRM, but the rising tide of computer logistics.
Having exclusive ability to repair their tractors does not make it easier for John Deere to stock that inventory. It makes it more profitable for them to stock that inventory, because they know they can sell it. That profit allows them to tighten their margins elsewhere, and to expand their belt elsewhere, but it does not fundamentally change the actual repair. It does make it easier in some ways that they can exploit supply and demand and economies of scale.
There are other ways that John Deere can enforce said Monopoly. They can sign contracts. Maintenance contracts are very common in this space, stating that you will only do business with John deere. That's not an unreasonable thing to do. I am a okay if every farmer wants to sign that contract with John Deere. If John Deere were to make that part of every purchase, at least it would be understood that it were part of every purchase, that you could not buy John Deere tractors without buying their service contracts, and how that interacts with existing antitrust law is a separate problem. One that has been discussed to death and will continue to be.