If you just start out selling tractors that are only equivalent to the incumbents and with all the same restrictions, who is going to buy them? The reason people want to buy from you instead of them is that it's possible to repair your tractors. And if people are buying from you, you're making money.
Maybe a few years later the investors decide they might like to make even more by closing things up, but by that point the software is already published for free on the internet (which was what got customers interested to begin with). So if they do all that happens is that someone else starts making repairable tractors, which people still prefer, using the software which is by then free and already exists.
Have people not noticed how much money Amazon makes out of "your margin is my opportunity"? Taking market share from the old guard by making less (but still some) profit per unit is a great way to make money.
There is known demand among farmers for repairable tractors, but it costs a moderate amount (more than an individual farmer has) to develop one. So you go raise some investment money to develop the software and then make it back by selling repairable tractors at a profit.
Maybe if you succeed you'll get copycats using your software on their own tractors, but by that point you've already succeeded. It may even help you by growing your ecosystem and making you more competitive against the closed incumbents.