I wish to therefore extrapolate this idea and attitude to the current controversy regarding conversions to Judaism performed in good faith and according to the letter of halacha. If decades later after the actual conversion ceremony, or even a relatively short time later, the convert for whatever reasons is not strictly observant of Jewish law or custom, does that invalidate the previous conversion ceremony itself?It seems to me to be self-evident that it could not and should not invalidate that conversion nor should the rabbinical court that performed the conversion be held accountable for the later lapses in observance of that convert. The rabbinical court that performs the conversion can only go by what it sees at the moment of the conversion.If it is convinced that the potential convert will lead a Jewish life and observe Torah, then it has fulfilled its obligation. It cannot peer into the future and know for certainty how the convert will behave in later life. It can only judge, and this is always subject to the errors that accompany every human judgment, the sincerity and commitment of the potential convert that stands before them at that time.Overwhelmingly, most converts remains sincere and committed Jews. But there will always be exceptional cases when it becomes obvious that somehow the convert has changed his or her mind-or at least their mode of behavior. It is a far stretch to try and invalidate the halachically valid conversion process because of the later behavior of the convert.
Has any Torah scholar ever complained about conversions to Judaism performed in good faith, the judges going by what they saw at the moment?! Of course, “a judge can only decide upon what he sees at the given moment when he renders his decision”. No, the issue is with those who close their eyes and pretend not to see what is obvious at the time itself!
Note: the fact that Rabbanut converts do not often observe Mitzvos later on in such fantastic percentages is merely a live demonstration of the sham occurring earlier, ‘belibo uvelev kol adam’.
(“Retroactive cancellation of conversions” is theoretical, to the best of my knowledge.)