Earlier quoted context omitted.
I can't help but think that this is intentional. I'm pretty sure it's possible to create a more modular design without sacrificing anything. Especially given John Deere's record, it doesn't seem far fetched that they would actively seek out ways to make it more difficult to repair their tractors.
It's definitely intentional. You pop the hood of a new car and you see a plastic cover, not the engine. The plastic cover doesn't do anything for the car, it adds cost to the car, and mechanics have to remove and reinstall it if they do any work at all. However, it does have a function: to obscure you from the workings of your car, and push you to take the car to a dealership or someone buying genuine parts at least.
My guess - the company that makes all the parts / wire runs / etc very visible has a harder time selling to mom and dad when they stop by with their kid.