Are you taking Nazi Germany as a example of what one can do?

Democracies succumbing to coups is a democratic process like dying is a life process. It's something the system is prone to doing sure, but after that, it is no longer the same system. To put it another way, eggs are prone to being made into omelettes, but after you do so, it is no longer an egg, and it is no longer capable of becoming a chicken; once you pervert a democracy, it is no longer a democracy, and is no longer capable of representing the people.

There's a sort of "reverse no true Scotsman" happening here because of a confusion of terms which creates the illusion of a contradiction. The claim was, "it is undemocratic for leaders to change the system in an anti-democratic way, despite being elected;" this is a claim about how democracies ought to work. Your counterclaim was, "this has happened historically in democracies" - a true claim about history, but these claims are not actually in contradiction and indeed both of them are true. Like how if I said, "cars drive on four wheels," it wouldn't be a contradiction for if you showed me a car which had three wheels because it had lost one in an accident.