I agree, that the Weimar republic was weak. Main problem was a missing election threshold, resulting in a divided, weak parliament. But some other minor things happened, too. For example several militaristic regimes packed on a small continent were competing to divide the world resulting in a world war, for which Germany was made solely accountable for and had to give up territory and pay huge reparations.
There are numerous examples of democracies collapsing from what seemed a stable state much quicker than that.
The collapse of the French Fourth Republic in 1958 is a good example[1]. In that case it led to another democracy, but that is only because the army wanted de Gaulle, and de Gaulle was happy to be president in a parliamentary system.
[1] https://en.wikipedia.org/wiki/French_Fourth_Republic#Algeria...