That's not because they were naive. You don't understand the state of Europe at the end of ww2 if you think this is the case. For the record, half of western Europe was still fascist at this time, and most of the countries would happily engage in an all out war to settle a hundred years old dispute.

The functioning of Europe is what it is because all European countries were very wary of the others, and very protective of their sovereignty. The only way to get Europe to exist was to have every member get a veto, basically. This way, each country can bargain what it gives to allow a common decision. And no country would be forced to do something by a more powerful country.

That's why it works.

Now, fascist anti-europe leader getting to power happened. It's not that it wasn't supposed to happen, Europe wasn't built to be future proof, it was built because it was a good thing to build. And it was built in way it could be built.

Now the question is how to prevent fascist anti-europe leaders to sabotage it. It is an important milestone. This would basically make Europe more of a federation, and I'm not sure most countries are ready yet. Even the supposedly good ones. Germany and France for example are quite fond of the power they have with the current way. Bribing Hungary is actually safer and easier than a system that could bit Germany or France back if other countries were to ally "against" one of them.