It's like if you're asking "what would prevent us from burning heretics alive again" or "what would prevent us from going back into the middle age again", or even "what would prevent us from buying slaves again". I'm not an expert of McCarthyism, I'm not even American and as such have not studied it a lot (barely read a wikipedia article about it and a couple of movies), I'd be more confortable answering "what would prevent us from going back into nazism"? Because I could there build an answer based on the current state construct and history.

But, I wouldn't construct my answer about "one thing specifically", instead, on a whole context of things, such as: evolution, experience, education...

It's a vague but extremely interesting question, I'm sorry I'm too tired to disgress as much as I'd love to but I suppose my answer would boil down to "we've made a long road since McCarthyism, we've learned to appreciate confronting ideas", it would be constructed about exactly why it's important to study history, keep a memory, and keep debating so that every single argument can be studied and refutated by anyone.

Again, I'm not fearing hypothetical realities that look like scifi, but instead, I'm trying to solve problems we actually have right here in our shared present reality, such as criminality. It's fine if you don't want to help me, that doesn't make you a criminal, but it would be great if someone like you with your talent would like to help.