The problem with this theory is that 4chan is older than both Twitter and Facebook.
If unmoderated speech created an inherently better platform, surely 4chan would have captured the market a long time ago and cut off commercial alternatives like Craigslist did.
1: Also as others point out, even 4chan moderates, however lightly.
Direct, unfiltered exposure to the firehouse is at best banal, and at worst disgusting and self-destructive. It's an _awful_ job that ~no one would chose to do for themselves.
It's the same reason people haven't mass switched to Mastodon or other Fediverse services; because the userbase is so much smaller than the likes of Twitter that there's a good chance the people and content they care about isn't available there. Or why so many competitors to popular services fail in general, regardless of their stance on free speech. The network effect is strong, and sometimes even billions of dollars and tons of marketing can't overcome that (see Google+ for example).
Would people prefer a free speech orientated alternative? Hard to say, for the same reason as whether they'd prefer a decentralised or federated one; it's the content and users that bring people to a site or service, and the competitors to the popular ones are so much smaller and less active it isn't much of a comparison.
