If you had a video or book or article 30 years ago, you couldn't even distribute it yourself around the world without the partnership of a major media company. It is way easier now.
Did people consider this a danger to free speech in the 90s? I certainly don't remember it that way. No one was worried that you couldn't get a main stream publisher to publish your crackpot theories, or that no cable tv station would air your crazy rants. No, you were forced to self publish, and deal with the fact that your audience was a lot smaller than if you could get a major publisher/media company to publish your work.
The NyPost still has a website, and anyone can still go to see it. It is way more accessible than self-published work was 30 years ago.
This is like not seeing the difference between allowing anyone to drive on a public road verse forcing everyone to pick up all hitchhikers.
I just feel that the issue of blocking a particular site is tangental to the real issue of the power these companies have. I am much more worried about the way their algorithms work for choosing who sees what, but that is not a concern based on 'free speech'
Social media is the tool, and some uses are ok and some aren't. I am not 'showing my true color' when I condemn someone who uses a knife to murder someone, even though I praised a master chef's use of a knife to cut food just the day before.
The government telling me, the hypothetical Twitter CEO, that I am legally mandated to publish your tweet, be it actually misinformed or just plain unpopular, or face asset seizure and jail time as a consequnce -- that is an _actual_, real infringement on free speech.
It just turns out that a lot of those SV libertarian free speech absolutists were actually partisan authoritarian hacks that are now abusing the notion of free speech in the current online discourse, so that the government will step in and mandate publishing their particular flavor speech. The people arguing against Twitter having the ability to silence and ban accounts (for any reason) are arguing for a Propaganda-class of speech that must be repeated by all who have any influence.
So maybe Twitter isn't quite a monopoly on its own, but the consolidation of control over information and data distribution into the hands of a very small number of organizations has worrying implications for how we think about censorship and freedom of speech.