> but also allow all content,
Almost no platform survive without a bit of moderation. If you don't moderate, you'll get any kind of content, including spam, troll, etc...
Add that to the fact that you'll get people that will just push to boycott such kind of platforms, and thus you'll no longer have much possible ways to make this kind of platform exist.
> but if someone posts something illegal, they take their share of legal responsibility for publishing it.
That's also kind of impossible. The law evolved to consider that impossibility to look at every piece of content, and this is why the DMCA exist. Look at Youtube which try to filter their content much further than the law currently require, they have HUGE teams of moderators, multiple tens of thousands, with some of the best kind of neural network, working on this and yet it fail so often.
The world isn't binary, we need a bit of both.
It could be an interesting experiment though to allow legally the kind of platform you suggest. Someway to protect website owner from any legal retaliation. It would most probably look like 4chan, but still interesting.