Way before that you could see that people became radicalized by consuming conspiracy theories far, far faster than any individual or organization could pull them back to sanity.
There's people making a living out of spreading dangerous lies to millions and millions spend their waking hours consuming them and not researching the opposite, the culmination was an abortive coup on the US Capitol, in between literally hundreds of thousands died in a pandemic where whole geographical regions were convinced that it was a hoax and that attempts to control it were a power grab.
If that's too little for you to accept someone's faith in unhindered free speech to be shaken, well, hard to find an angle to argue from, it's a hard absolute you're presenting.
PS.: And also how Parler is responsible for any of this would be interesting.
Parler was a place where people discussed these things leading up to an abortive coup or revolution, however you want to call it. The US clearly doesn't have the means to control these things as of now, Big Tech taking the matter on its own hands is merely symptomatic of the lack of legislation preventing the spread of radical fascist ideology.