Eventually, wealthy and liberal societies come to an end for other reasons
Those 'other reasons' are pretty simple: Liberal and wealthy societies become complacent due to the ease of their lives, and that makes them neglect the principles and practices that made them powerful and wealthy to begin with.
The default human condition is poverty, misery and violence. Escaping that is rare, and it takes a special society to make wealth, power and security seem normal. Once wealth, power and security are seen as birthrights and not hard-won prizes, the parts of a society that make it special are neglected (because, hey, they're 'mean' and 'hard work'), and rot sets in.