Consider the argument of 99% of free speech flowing through 3 companies. Not corporate free speech, but regular people speech. Public speaking in the town square has shifted much in the same way most other faucets in life have shifted, yet fourth amendment protections still protect you from warrantless searches on the internet. Most things have shifted since 1787. If we don't have protections in place for that speech, then you have an absolute easy/legal time establishing a Corpo-state ala modern day China, something antithetical to the Magna Carta / English bill of rights where the first amendment is cribbed from.
The Covid pandemic really crystallized it for me, as people like Dr. Chris Martinson got deplatformed for stating basic facts that ran counter to the current CDC guidance; CDC guidance which was always a few months behind the raw science but eventually caught up.
Here's one somewhat less political thought yet still should have been not bannable off social networks -- The lab leak theory. The common narrative of the lab leak theory is it's 'racist', thus leading to deplatforming, yet it continues to have more and more supporting evidence.
With the assumption that speech is public forum if the vast majority of speech occurs that way, I don't see how the issue impinges on others rights by protecting it. Online speech provides the same protections as current speech does, in the terms of you don't have to listen and agree with someone speaking about something publicly. You can block them yourself; or you simply never see them anyway because the search algorithms are so tuned to feed your filter bubble anyway. Corporations already have sheltered immunity from the speech of others, as long as they don't act as a publisher would. The direct costs of the speech on these platforms is probably even lower then public speech in the common square, given the maintenance and traffic control costs one would have with a 100 people listening to someone speak in public. So why not let someone speak if they want to on a subject digitally no matter how horrible and contentious it is to you?