I'm pretty sure a lot of work has gone into making institutions resistant to a potential future super-Hitler. Whether those efforts will be effective or not, it is a very real concern, and it would be absurd to ignore it on the grounds of "there is probably some limit to tyranny we're not yet aware of which is not too far beyond what we've previously experienced." I would argue a lot more effort should have gone into preventing the original Hitler, whose rise to power was repeatedly met with the chorus refrain "How much worse can it get?"