If I understand correctly, there are even people now who will claim that if one ger converted by a rabbi goes off the derech, then all the gerim ever converted by the rabbi are considered invalid converts! Madness.
Actually, I think that is the least mad way of doing it.
Because, I imagine the determination goes by what was in everybody’s minds at the geirus. So that devarim sh’believ einum devarim, and if the dayanim made a proper determination, then anything that happens subsequently is not relevant. So we must be deciding that this dayan doesn’t make proper determinations. So then the dayan is the one who is passul, and of course you can’t rely on any of their geirus.
I really think this whole issue is overblown anyway. In 20-30 years, there will have been a decisive split, with Avi Weiss and co. going one way, and orthodox judaism going the other way.
That’s the way Orthodoxy works in the past couple hundred years. Every other generation, you need to clean up the ship.