I agree with you here.
However, Reddit and similar platforms usually can't operate under this logic. Why? Because they need to make money to stay online/keep operating. And unfortunately, that means content that alienates advertisers (like a lot of the more controversial subreddits) ends up actively hurting their revenue stream.
They could try and get round this by monetising in other ways (micropayments, subscriptions, purely donations, etc) or by looking for ad networks with real low standards, but those methods likely wouldn't pay enough to keep them afloat.
It's why in the long run, I suspect the answer to the problem isn't a centralised platform that hosts a lot of different communities, but a system that connects individually hosted communities into a 'network' of sorts with similar features to Reddit.
That way, you can have both controversial and non controversial 'subreddits', with the latter paying their own hosting bills and the former either doing the same thing or getting subsidised hosting by the network owner.
It also shuts down any future attempts at censorship by not giving the company running the service any way to edit what happens on any one community.