Then what? We probably see higher volatility due to an increase in crop failures, but maybe lower average food prices overall.
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.
That's weird, cars get by just fine on having mechanics in every town.
They can more than afford to offer a high level of service at harvest time without ripping off their customers and screwing over everyone else as well by opposing our right to repair.
As it stands their current level of service isn't even that great. Farmers have been complaining about not being able to get parts (https://talk.newagtalk.com/forums/thread-view.asp?tid=104450...) and saying that because they're forced to get repairs from John Deere that single source for service gets overwhelmed and they can end up waiting for weeks on technicians while watching their crops rot away. (https://www.ifixit.com/News/62788/john-deere-has-been-violat...)