Perhaps Parler wasn’t a true free speech platform, ie one that moderated only the legal bare minimum. But, even if it’s true that it was friendly to only end of the political spectrum, at least it was a competitor to Twitter. The elimination of the sole significant direct competitor to Twitter, even one that represents a subset of views and not all speech, is the problem. It means there’s a tech oligopoly.
I’d love to see a decentralized free speech platform. I’m no blockchain evangelist, but it would seem like such technology could be used to build a real online “public square” as you say.