You are of course free to create a platform that only removes illegal content, just as Twitter or YouTube is free to to remove content they don't like.
But I think you'd find two problems: determining what could be illegal is really hard (what's a "true threat" and what's a tasteless joke?), and also you'd end up with a community that looks a lot like 4chan or parler. Not a place I would choose to hang out.
While that may be true.. do you not believe that the exceptional openness of either of those two platforms has an impact on places outside of them? Do you think there's no intangible benefits to you by these places merely existing?
The primary export of places like that has proven to be brigading, harassment, and bigotry in the form of memes, so...no, not really.
I have yet to hear a positive argument for their existence, and "well it collects the dirtbags!" is actually not one. I spent years tracking reactionary and fascist movements on the internet and how they interrelate and spread information; these sites more or less exist to do exactly that. The targeted harassment campagins that target random people they've decided not to like--that's just "for the lulz".
The norming of bigotry should be recognized and acted against when it occurs. When that norming is done through (shitty) humor, it should be pointed out as such. Did you have a point to make?
Many well regarded distributed platforms are like that too (the web and blogosphere, email, mastodon, xmpp&matrix). The common feature is federated islands that can somewhat form filtered views of the global system.
I think we have proven time and time again that you can not actually create your own platform, if you try then come after your hosting providers, your network provider, your advertisers, your payment processors, and if that fails they find your family to go after their employers, etc etc etc
So the "just create your own platform" trope has be tried and failed.
But I think you'd find two problems: determining what could be illegal is really hard (what's a "true threat" and what's a tasteless joke?), and also you'd end up with a community that looks a lot like 4chan or parler. Not a place I would choose to hang out.