You can fix things, but can you really justify the time to do when you need an operational machine?
1. No matter how great of a shade tree mechanic you are, you will never be able to fix it faster than someone who does it every day. They have found all the little tricks and quirks about your machine that your casual maintenance will never uncover.
2. While large farms with full-time mechanics on staff have been known to make deals to warehouse parts in their own shop on consignment, much more realistically for any kind of normal farm you are going to have to drive to the dealership to get the parts you need. Whereas the dealership tech can bring the parts to you. Meaning that you have to travel twice as far, taking twice as long, to get the parts back to your equipment than if you call a mechanic.
The things that are likely to fail under use where there has been proper maintenance tend to be the things that are unpredictable and catastrophic, at very least requiring parts, and most likely requiring advanced knowhow. And at that point, the dealership tech is going to be faster at getting you back up and running, even if you could theoretically pull it off yourself. So, realistically, there isn't much of a compelling case for doing your own repairs when time is of the essence.
Farmers are often willing to accept more downtime to do it themselves out of pride, though. I admittedly often fall victim to that myself, so I get it. But it’s clear that the farmers who are serious about farming as a business aren’t dinking around trying to fix things themselves. It is not economically prudent to do so. Granted, not all farmers farm for business sake. For many it’s more of a hobby or lifestyle and wanting to be a part-time mechanic can play into that.