I did a lot of construction until I was about 25 and even some welding. The vast majority of guys working at the factory are welders. I did a lot of ag construction including building a crapload of Sukup grain bins to name drop one of our ISU athletics benefactors. I could never get the hang of welding. I suck at it.
You can't learn to weld in 2 hours, not the quality level you'd want. You can't learn to weld to the degree you'd want on high quality farm equipment quickly. That's just my opinion of course but there is an art to welding and if it were just screwing nuts on bolts you'd have a point but you're wrong. Just my opinion obviously.
Also you aren't replacing that number of workers easily. That's just false.
I really doubt a vast majority of the people working at the Deere plant are legit certified welders. If so, they sure as hell aren't the ones supposedly making $18-20 per hour. If you're a certified welder and working in a factory for $18/hr then you are a complete idiot.
We've had students that weren't certified but had some ability to weld, and we can't keep them because they can go make $25/hr or more part time while going to school.
It probably depends on the line, but I bet the amount of legit, manual welding is tiny.