Not so fast! What about A1200s and A600? Their motherboards are the same as peecee motherboards, because everything except the Kickstart roms are surface mounted, and most of the chips are the ones with those tiny pins. Just try to repair that.
If some caps break, then you'll be able to change these on a peecee motherboard easily enough.
Bottom line: It's mostly the same kind of technology, and what you can and can't repair relies purely on what's broken, and whether or not you have the skill to fix the problem.
The 4000 uses the same SMT chips .... that didn't stop me fixing it.

The point is that Amigas have standardised parts that are the same in all machines, I.e. all AGA machines use Paula, Alice etc.... so there's lots if them about. With a PC, you need to replace a particular part that will have been used a lot, but also thrown out because they're commodity parts... and it's very difficult in such a fast changing market to find a donor machine with exactly the right replacement part. To repair an A4000, just buy an a1200.
Not only that, but people are designing FPGA replacements for Amiga chips, but who's going to do that for a VL82C107FC or something?
Replacement parts for Amigas will be available long past the parts necessary to keep ok PCs running. It's not the skill to repair a machine that's lacking, it's the parts. Anyone can learn to repair machines, but very few have their own manufacturing plants! Thank goodness for FPGA though, this way there its hope even when thee originals have all gone.