there are many users with pcb s5 fails, then they could sell to these users and make profit
"many users" = not many users out of 10,000s of S5s sold.
Also it was mentioned a faulty one. So it would be a crapshoot. Bitmain might get something that is junk and would take more money to repair then it's worth.
Bitmain is not going to go to a model you ship them old parts and they pay or give compensation. Dogie is right about small overall Also keep in mind when they mad them they did it in MASS quanity so the price they paid might be quite a bit less then you would want for a pcb anyways.
If they felt there was a big need they would produce them again. That is just how it works.