With this sentence, you have completely shot holes in your own case.
How exactly is a customer to deal with the results of a poor tech when hundreds of miles out to sea, or even a few miles or so out in the stream?
As for the internet weeding out the poor techs, I should think that it would be 100% apparent the role it could play.
When I was not operating a vessel professionally, I was a service tech in Fort Lauderdale. As I had no shop, just knowledge, and tools, I was a no cure-no pay operator and very, very much in demand, especially by the yacht brokers who knew they could rely on my getting the job done or not, but not BSing anybody.
As for, "On the one hand, I hear some claim "It ain't brain surgery", or "rocket science"; yeah I like that one (even though the work is a craft in a science that is identical to that used on rockets)", I don't think I have ever heard any owner or service tech (good or bad) in over 5 decades of being professionally involved in the marine industry, make this statement about anything other than the physical act of sailing! Marine systems are complicated, especially compared to the same system ashore. You turn on a tap in both cases to get water to wash dishes, but on the boat, there must be a tank, a pump and some sort of pressure sensor to maintain the water flow. On land all that is done far from the consumer.
Furthermore, a qualified and experienced marine tech should, in his own field, definitely be familiar enough with the equipment he purports to be able to repair that 15 minutes would be sufficient, in my professional opinion, to be fairly well into the diagnostics. Heck, I walked aboard a completely dark Danish (every label on the vessel was in Danish, a language I did not speak or read) refrigerated freighter with a helper (who also did not speak or read Danish) and had every system functioning in two days. It took about four hours to get the first generator operating and after that, it was just a matter of relabeling switches through trial and error to get everything else up, labeled and operating, including 4 ammonia refrigeration systems for the cargo holds.
I've found almost all yacht manufacturers use similar, if not the same exact systems, depending on the age and place of the build (pumps, electrical panels, lighting, appliances etc). For instance, there are very few marine refrigeration and air-conditioning manufacturers and any reasonably competent tech in that field should be able to analyze and repair any one of them, pretty much without even needing to read the tech manual.
Too many service personnel today, in all industries, not just the marine industry, have absolutely no comprehension of how the equipment they service works. They simply change parts until they find the one that caused the fault, and the customer must pay for everything that came before the correct part.
A truly competent and capable service tech can weather the occasional poor customer, but a poor service provider is a danger to those he services. You just cannot compare the two.
I agree that an excellent service provider can weather the odd bad customer...
Just as a competent skipper can weather the odd bad service provider...
...by performing their own due diligence and not hiring them in the first place.
Additionally, a competent boater can ensure that the service contract includes a method of inspection / test / verification before leaving the slip.
Quite frankly, I insist on it as the service provider, to protect myself from the boater.