It was in stable condition until it was set on fire by a westerner and until part of the remains were stolen by other westerners. Plus the country would be a democracy today without western 1953 coup.
Xerxes shouldn't have burned Athens. His father, Darius, in fact had advised him to follow the protocol set by himself and Cyrus.
Dr. Mossadeqh was not running a "democracy". Post WWII Iran's political space was far more complex than the caricature presented since the fall of the Shah, and sans British instigated support for counter-coup to remove Mossadegh with help from USA, a quite significant chunk of Iranian military and society, including the Clergy (who were already using terror in Iran, btw), agreed with the American analysis that Mossadegh would merely precede a Soviet controlled Tudeh takeover of Iran.
Persepolis wasn’t in great condition when I went but it also wasn’t a shambles. There’s a gift shop, guards, and a reconstruction of part of the palace, and a lot of tourists. Also a small set of offices for archaeologists. I’m a professional historian so I was a little bummed by the lack of adequate public-facing educational markers and texts, but it certainly can’t be compared to something like the Buddhas of Bamiyan as an excuse for removing items to European museums.
https://en.wikipedia.org/wiki/Buddhas_of_Bamiyan