Is it problematic that people take social rejection as a reason to further radicalize? Sure - but such is human existence - and where is their personal responsibility to observe their social environment and alter their behaviors/beliefs if they are unhappy with the results they are receiving?
But from a policy perspective on the ground, I think the ideal of freedom of speech should eclipse the concerns of private platforms wherever possible. But this is not to say we should not police the boundaries of free speech. In fact, if we had a more strict enforcement policy about unprotected speech like direct threats (such as L. Lin Wood's declaration that people should execute Mike Pence by firing squad [0]), we might not need these platform interventions at all.
[0]: https://www.msn.com/en-us/news/politics/secret-service-says-...
Maybe we need a "C-SPAN/NPR of social media".
Let's say if these companies are monopolies. Should they still hold the same power?
The main exception is utilities which generally operate as natural monopolies. They should be (and generally are — this is isn’t a new idea) regulated to only be allowed to deny service for limited, specific reasons. There’s room for improvement here. ISPs are relatively new to being treated as utilities and many ordinary regulations that should be in-place are missing.
First, election cycle is over; second, they are not banning one political party.