First, I see you are not an engineer! That is clear to all!
You may be assuming that because you are comfortable with fixing computers, therefore everyone is? It is great that you are competent, but it is a mistake to assume everyone else is equally competant...because most are not!
you bring up to separate issues:
1) The vast majority of people will never even think of opening up their machine. They'll throw it away if something starts acting up...even if it is a stupid easy repair.
2) The people who work at Apple stores are low skill and have no experience. I hear turnover is quite high. So bad/ wrong diagnosis is a problem due to inexperienced people, but it isn't some kind of conspiracy.
3) in all service scenarios, in all industries, it is common for techs to replace perfectly good components. I have trained myself to test and re-test to make sure I have actually found the issue, but that can take a lot of time. Mistakes are made, because people don't take enough time. It is a people problem, not an Apple problem!
4) Isn't your example of a loose connector proof that soldered in is better? If that connection had been soldered, it couldn't have come off...and none of the failure and mis-diagnosis could have happened!
Have a nice day!
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If soldering components is bad, where exactly do you want them to stop soldering? Should they have sockets for every resister and capacitor? Should they build their machines on a breadboard with through hole components?