You are forgetting that many faults are from more or less design issues on the particular product revisions so there is a lot to be said for identifying the fau

You are forgetting that many faults are from more or less design issues on the particular product revisions so there is a lot to be said for identifying the fault pattern once, then just applying it over and over again. Then, the cost you are competing with is the replacement cost for typically the entire logic board, which can easily be $800 - that buys a lot of hours. That said there is a kernel of truth here which is that component level repair requires diagnostic acumen, creativity and some mechanical skill. That requires some actual expertise, training, .. in your repair business instead of hiring monkeys that replace full boards no matter what.