I know for a fact that there are competitors, in Europe we have many other brand of tractors. It would make no sense to buy something that you know you can't repair.
I know a few farmer in Europe, despite being better if you tell them they can't repair their engine, they would get very angry and never buy this brand again. When things break its faster to repair themselves because they already repaired it many times.
But here farms are much smaller than in the us, so it might be a matter of priority. If you have so much land than loosing a day on repairing something makes you lose more money, it makes sense to go with John Deere.
On the other hand, these farmers all have several tractors and old equipment like 40 years old so that when a thing break they can still use another even if less efficient to do the job.
At some point all corporations start to trim expenses or generate new revenues from existing avenues - so that reputation was used to get everyone kinda trapped in their JD world that was essentially made "unrepairable" overnight.
This is the same thing Monsanto did with seed, Dow chem with Roundup and Microsoft with windows for a non farm example.
Once you envelop an ecosystem of products and services the people within that system are largely at the mercy of it. Apple and their batteries intentionally being made to fail scandal a few years back is another great example.
It's just the numbers are much bigger in the agricultural world than the consumer tech world. The tractor that has essentially a SaS contract/required yearly operating expense for maintenance and upkeep might cost millions in the first place. Many farmers have machine shops that are plenty capable of repairing tractors - bc they've always done so.
JD is trying to fundamentally change the game - it's not the farmers fault for being a loyal customer for 10-40+ years, as many farmers today inherited farms that already had JD equipment from their fathers/grand fathers.
JD is exploiting that and their supply chain to make tractors and farm equipment glorified rentals.