Here's a video where they remove and put back a MacBook Air motherboard in 13 minutes, on camera (https://youtu.be/Sk8kj_y32mU?si=4fUoXux_zLIGg-9B).
I already took into account a pessimistic 50% repair success rate at a salary double what most shops pay.
So then you have another 20 minutes to diagnose. If you can repair 50% of boards that make it far enough that you need to dissassemble for diagnosis and then you take an additional 30 minutes to do the repair with 50% success rate, you're spending 3.6 hours per repair, which is perfectly fine if you're charging 300$. These are pessimistic assumptions too - you often don't need to dissassemble the board to diagnose it, the person disassembling the board doesn't need to be paid as much as the person doing the repair, and you'll probably have a better than 50% repair success rate.
Total bullshit. It takes more than 20 minutes just to disassemble and reassemble a laptop to get the motherboard out without damaging anything. Diagnosing the failed component can be quicker in some cases (eg. scorch marks), but checking the extent of collateral damage isn't that quick. A more realistic estimate would be about two or three repairs per day, unless you want to complicate things by stipulating that all the less skilled parts of the process (disassembly, fetching replacement parts reassembly, verifying repair worked) are to be done by a less skilled technician and only the actual board inspection, probing, and rework are done by the skilled technician. And then you have to consider that such repairs will never have a perfect success rate, so you need to be able to regularly eat the cost of wasted time on failed repairs while still keeping the cost of labor and maintaining an extensive parts inventory cheaper than the cost of a new motherboard.