To look a population studies you don't just draw conclusions from where it happened to work, you look at the whole population and see the number of places where it didn't, and ideally compare places where take-overs were done within the systems.

Oh, Hitler was elected to power legally.
And look at Africa. The number of leaders who have extended their terms in office or just removed the opposition parties show that usurping power from within democracies is alive and well.

Slavery in Europe / USA was got rid of as it was banned
The Tugee cult was destroyed as it was banned and hunted out of existence.
Pirates were removed from the sea not by debate, but by blowing them out of the water (we're trying the debate method at the moment - and it isn't working[!])

I could go on too.

It's easy to see where things haven't worked, a lot harder to know when they have and hence nothing happened.