I would think that once an assembly line has been set, full automation would be completely ideal and perfect. Is it because plants are always changing and getting tweaked, and changes require software updates, which invariably have bugs and take time to adapt to changes?
A robot will uncritically do what you tell it.
An experienced welder will tell you you're an idiot when you're an idiot.
You could imagine a situation where the automated QA is trusted and does its job in sets, with humans interspersed to backup the QA programs. But one of the QA checks is actually buggy and OK’s the missing weld, but the full batch of work covers it up before reaching a human for the double-check
In which case, both issues are at fault: too much reliance on the QA program, and allowing too much work to be done before it reached a human (bad double-check setup)
Especially in case the robots share code and e.g. have the same parse error when loading your list of welding points.
And might well muck it up one time out of 50.
When you have humans running a line, they can notice and adapt to the countless possible issues that occur (like "the alignment here is often slightly out" or "tightening X here causes problems for the people in the next station"). Their top speed is theoretically less than a automated line, but they are quite adaptable and robust.
Since automated lines are less adaptable and robust, so you have to get them just right. Any tiny little issue can cause surprising problems down the line and implementing fixes is quite slow because you can't just describe the problem and required fix to a human and leave them to work it out.
I think the best way to set up a production line is to exclusively use humans at the beginning, and carefully introduce automation once the process is understood and stable
That being said, car parts in general seems so unnecessarily complex. Too many curves, too little compartmentalization and modularity, etc. At this point in history, I would expect the average person to be capable of practically disassembling most cosmetic pieces in under an hour and with little research, but it's an obnoxious process of hunting down hidden screws and plastic clips in precise sequences. If cars were built in a reasonable manner, it should have been trivial to repurpose most of the parts in these vehicles.
However, I do believe if you were to design the vehicle for optimal automated assembly throughout, it could significantly change the situation.
But a design optimized for 100% automated assembly is probably not going to be as servicable. Imagine for example if wiring harnesses were built as rigid modules which were captive within the unibody when it was welded together. This could probably simplify the automated assembly, eliminating a step of snaking a flexible bundle of wires through passages, but then nobody is ever going to be replacing the wiring harness module.
I do think things can be done in the vehicle to improve the automation without compromising servicability, but it might not be economical, or maybe it just requires more control over the entire supply chain to tailor everything to the vehicle's assembly process.