The point is not to get a cheap tractor, or even a good one.
The point is not to have a tractor you can service.
The point is to have a shared platform.
If you buy a random used tractor and mod it, and I buy a different one, the probability that your mod is compatible with my tractor is low.
If I am just as skilled as you at designing and fabricating parts, maybe that's ok.... I can just adapt your plans to my machine.
With open source hardware, though, you can be a skilled machine designer, release detailed plans, and then a less skilled designer can come along, follow your fabrication instructions, and have a chance that your mod will work on their hardware.
Open sourcing the plans is essentially providing an standard API for the machine.
If you are satisfied that the existing market for farm equipment gives you an optimally efficient process already, then this is of no benefit to you.
If you are a tinkerer and would like to build off a community of tinkerers to design new workflows for your operation, you might find an open source hardware project valuable.