Syria was an absolute hell under Assad for dissidents, can't blame America for that. Iraq and Libya maybe, though Saddam and Gaddafi weren't exactly great leaders to their people either.

Anyway, IMO the thing about Iran is that it's mostly Shia, and the population isn't that religious, especially not in cities. Unlike Syria, Iraq and Libya of the past, they aren't ruled by a secular dictatorship, but religious extremists. So, while US intervention in Iraq, Libya and so on created space for religious extremists to rise, I think getting rid of Iranian government could actually do the opposite - give a chance for secular opposition to rise.