Could a rebuilder/dealer feasibly stay in business for 2 generations without being any good?
the short answer? ... yes
That is of course not to say that the rebuilder in question isn't good, but the sad fact is that many people are incapable of discerning good rebuilding work from bad, and many rebuilders do cheap work and stay in business for a long long time.
I don't know how a non-piano expert can tell this. This is one reason I feel that trusting the person you work with is so important. How can you learn about scale design, and soundboard construction? How can you in a few weeks learn what a good rebuild and what a bad rebuild look like? When an action has all of it's parts replaced, how can you the non-piano expert know whether they were replaced properly? The capstan screw line being off 1.5mm can drastically affect the performance of the action for the worse. But how would you ever know that? It has new parts, everything looks clean and properly alligned...
Same goes for finding a technician.
I would ask people who seem to have high standards in other areas of life. Find someone whose judgement you trust in other areas, and ask them who they recommend as a piano technician. This may be a teacher or a professor, but someone who is not just good, but strives to be the best.
Have that technician look at the rebuilds you're considering.
That's one way to do it.
But I still recommend finding someone you trust implicitly, and take their word for it.
Finally, let your ears and hands be the judge. Does the piano play like a truck? It shouldn't...
Does it sound unneven across all registers?
How does it perform? Does it feel consistent at different velocities from note to note?