Hypothetically, let's suppose that the entire Internet was controlled by one private company. (This is very plausible - I see no reason why couldn't have developed this way if history had gone slightly differently; in fact I think it's a miracle that it didn't.) Would you be okay with that private company unilaterally deciding which political opinions were worthy of publishing, even if it was their full legal right to do so?
I'm not a free speech absolutist and I fully agree that not everything that people publish on (say) Twitter should be allowed to stay on Twitter. The problem is the growing centralisation of power and the lack of democratic oversight. One day everyone who currently works at Twitter will be dead, but Twitter may still exist, staffed by a completely different bunch of people. Can you be so sure that the reins will always remain in hands you approve of?
I don't know what the solution is to the problem of centralised private power to control the internet, but it baffles me that anyone could deny that it is a problem.
The problem you proposed strikes me as primarily being that one company controls the Internet. That's the problem that needs to be addressed. It might have implications for the speech issue you're suggesting, but I can safely punt on that. Break up the single monopoly in order to reduce the degree to which any one private actor acts as a pseudo-state.
A counter-example for you is the following: suppose that a million companies control the internet, and all of them independently decide they don't want to publish white supremacist content. Then the problem is clearly not concentration or monopoly or collusion, but rather companies (via the people who make publishing decisions for the companies) exercising their own free speech. I think the challenge is that in this counter-example, the free speech absolutist -- not saying you are one -- needs to admit that they want to restrain one sort of speech (not wanting to publish white supremacist content) to enable another sort of speech (people publishing white supremacist content). It's an essential tension. Someone has to lose in that exchange. We need only clarify who.
It may be the case that the internet is far closer to your example than mine, though I doubt it. Even if market share of social media suggests concentration, it seems to me easier than ever to publish your content. I published my first website in 1995 and surely I have an easier time doing it today than then. Now, I might not have an easier time getting an audience, but I am also not entitled to an audience. If people don't look for me or tune me out, that's not a violation of my rights. That the public has chosen to waste less time seeking out dissenting views is maybe something we should be worried about, but probably not something we solve by legislatively privileging access of dissenting views, no matter how nuts, to platforms.
I just wanted to add one final thing. Matthew Lyon was a congressman. When John Adams passed the Alien and Sedition Acts, Lyon attempted to publish a piece calling Adams a dictator and a tyrant for doing so. The local newspaper, the Rutland Herald, would not publish it. So Lyon started his own newspaper. Lyon was subsequently convicted and jailed. In this example, we see three things: (1) the government jailing Lyon is a restraint on free speech and unconstitutional. (2) The Herald not publishing his letter is not a restraint on free speech. (3) Lyon did what we would expect people to do: work to amplify their own voice when others will not.
Today it is easier than ever to "start your own newspaper". You might find that no one reads it -- but you are not entitled to their readership, just as Lyon was not.
> The problem you proposed strikes me as primarily being that one company controls the Internet. That's the problem that needs to be addressed.
You don't forego treating someone's symptoms just because you don't yet know how to cure the disease. You need to manage a patient's symptoms and keep them alive.
Figuring out how to unravel or limit these huge monopolies is something that will take government years to figure out, if ever, much like finding a cure for a disease. As much as we need to solve that problem, we can't just throw up our hands in the meantime and say "I guess there's nothing we can do."
> Can you be so sure that the reins will always remain in hands you approve of?
Actually, when one deliberately obtains the power to design the audience those who helped to establish this are by definition not the design you are looking for. At first the useful idiots must be portrayed as model citizens and as majority opinion (by silencing everyone else) then they too need to be silenced to preserve that illusion. You don't want subjects who have reason to praise you, you want ones who praise you because they are your subjects.
A website is exercising their own freedom of speech when choosing not to publish or carry something.
If we oppose this idea then we need to create an alternative to that monopoly. Either in the private, or the public sector.