Well, it's not that hard to imagine, is it? Someone founds a company that produces tractors but they are simpler, cheaper and can be repaired. If that's what customers want, they _will_ gain market share. If not, then the demand isn't there.
Then the next argument would be that John Deere is destroying competition by artificially selling their tractors below cost, making it impossible to compete with them.
Because yeah, that's what you do when you recoup the rest of the cost in the subscription/maintenance.
It's the same model that game consoles use to keep the hardware costs low. They make it up in the game and peripheral licensing fees.
If JD dropped the subscriptions, but instead charged full price for the tractors, people would just scream about the prices instead.
I actually agree that farmers should legally be able to repair their own tractors, or have anyone else do it. But gaining that "right" back wouldn't actually fix their complaints, just shift them.