People are already hacking JD equipment with firmware found online. I don't have an exact answer to your question, but this thread seems over-blown. Somebody will know how it works, or learn how to work on it. I trust the ingenuity of this nation wouldn't leave us with an unmaintainable or unhackable tractors without "John Deere" the business existing, in the same way the technique for jailbreaking the latest version of iOS comes relatively quickly. The country that went to the moon can reverse engineer a tractor. It's not like we need to prove P = NP or not to start getting people fed again, to fix the problem.
I personally agree the problem's being overblown here, and I agree with what you say about technology like that, but, just to counter a bit, when it comes to agriculture, time's not always available. Depending on when that theoretical stopping of Deere's servers were to occur. There could be massive crop failures. It's all well and good to get everything back up again, but by this time a loaf of bread costs $50 and everyone's flipping their shit.
Not that I think any of this is ever likely to happen, but even small disruptions to the food supply can have large consequences.
