You mentioned someone who claimed that democracies were necessarily permanent in nature. It’s not an ad hominem to point out that he’s been wrong for 250 years which is a about as close to permanent as you get in government terms (and certainly he wasn’t arguing that democracies like all governments are not infinitely self-perpetuating, since no one claims they are). That someone said something couldn’t happen and was wrong about it is an argument against the thing he was arguing. If you’d like to argue that he was wrong then, but somehow now his argument holds water that’s fine, but why would that be?
I also can’t really imagine how you can square the statement you quoted (that democracy cannot be anything but temporary) with him only opposing utopian democracy.