I want this, my neighbour wants this, his neighbour - who runs an older, pre-proprietary John Deere wants this. Farmers need their tractors to work and anything that helps there is a boon. While ag contractors may run the latest most modern equipment farmers tend to have a few tractors themselves which tend to be a bit older, a bit more run-down than those shiny new JD/MF/NH/Valtra (in Sweden and Finland)/etc. machines. They can still do with some of the nicer parts of the electromagic on those machines.
So... once those JDs go out of support, who knows? There may be a market for an aftermarket control box, one which pulls the machine out of the clutches of the mothership and puts it where it belongs, in the hands of the farmer. I have a proposal, just donate one of those machines to me and I'll rig up such a box. Not that my current 1982 UTB640DTC (a Romanian license-built/copied (it is a bit unclear) Fiat 640 with FWA and a front loader) has given up but still...
So, good servicing is fine if you can afford it but many farmers tend to their own equipment as well in which case the availability of parts and protocol specifications really comes in handy. Mechanical parts - that what used to define a tractor - tend to be readily available but electronics and specifications are a different story.
I think you really hit the nail on the head there. In the brave new world of tractors that actually have (lots of) software in them, we need to make the right comparisons/analogies.
Saying "no farmer wants this" is the equivalent of saying that "no farmer wants the technical manual" for their pre-software-filled tractor. I bet it helps tremendously if you have the manual for your 1950 International Harvester Farmall MD and you want to keep repairing it. I have no idea how to repair tractors, nor do I own one or will probably ever own one but for some reason watching this was fun: https://youtu.be/7yuHIu1IfPw?list=PL-1mGLAjHPWy0BL-MO6MrSr-m... And somewhere in there (or maybe somewhere else on his channel) he shows these manuals, where to get them etc. Apparently even though some of the companies are out of business or no longer provide these themselves, there are re-prints of these and other companies that still make parts for these engines and tractors. The guy has a pretty small operation it seems and uses tractors mostly to make hay for his cattle but it looks like he's using that 72 year old MD as well as some other slightly (!) newer tractors to run his operation.
Farmers don't want an open source tractor protocol for its own sake, but if it allows for greater flexibility, reduced costs, and improved serviceability, then those will be relevant competitive advantages. You can see a similar phenomenon with linux on the desktop; Most people don't care about linux being open source in principle, but there is a meaningful, if small, amount of mainstream interest in it for reasons such as privacy and extending the lifespan of older hardware, both of which are enabled by linux's openness.
No farmer wants this. Good servicing beats open source in hardware, in farming and frankly most non-enthusiast spaces.