are gezeiros and takanos irrevocable?

The Rosh (Kesubos 4:24) quotes a mesorah from the Geonim that they would no longer convene a Bais Din to pasken collection of kesubas banin dichrin. The whole purpose of the takanah of banin dichrin was to ensure that a father would give a proper dowry to his daughter; by instituting that the kesubah be inherited by grandchildren and not by the husband and/or his family Chazal felt a father would give more freely. The Geonim saw that in their times large dowries were freely given and the takanah was no longer needed.

Apparently the Geonim did not place much weight on the counter-argument (advanced by R’ Yonah) that a takanah remains in full force even if the reason behind it is no longer applicable. The idea that takanos are irrevocable is taken as common knowledge in yeshivos based on the rule that a later Bais Din cannot revoke the work of an earlier one. The usual explanation offered (I think based on GR”A) is that takanos are overdetermined - based on multiple reasons - some revealed to us, some hidden, so that even if the given reason is no longer applicable, others may still apply. In fact, there are probably as many exceptions to this rule as cases to illustrate it.

My son recently asked me about people clapping and dancing on Shabbos. I pointed him to Tosfos’ comment (Beitzah 30) that these gezeiros were formulated lest people come to make or fix musical instruments, and since the reason no longer applies, the gezeirah is no longer in effect. Achronim struggle to make sense of this Tosfos, as Tosfos clearly does not assume a takanah is binding if the reason behind it is not applicable. Again, the exception is striking only if you see the rule as absolute. In one of the Maharatz Chiyus’ essays he rounds up a collection of these cases and tries to come up with a pattern that fits - something to keep in mind when these cases come up.