Should a platform be coerced into allowing harassment and hate speech just because that happens to be someone’s political stance?
Hate speech is not something you can nail down, because it's not a real thing. The working definition of hate speech, as it's actually used is "Anything I don't like". I could very well say that you are being hateful against right wingers (I'm not saying that, but I COULD), and it would be about as sound an argument as any other for hate speech.
So yes, if a platform has a monopoly, then they should be coerced into allowing "hate speech".
EDIT: I should probably make it clear that my preferred solution is to force (yes, force) Twitter and Facebook to make themselves compatible with API's like activitypub or similar. I understand that's a pandora's box, but we seem to be in Pandora's emporium in this situation, and must open some box or other.
There is no justification to force someone to make available what someone else uploaded on their servers.
From a practical level, the internet is a new medium, and were constantly transitioning from one state to another, and are far from settling down yet. Therefore, any rule you make will be out of date a few years down the road.
And I say this as someone who doesn't believe “hatespeech” should be censored or punished in any way.