It's not a lazy argument, you're just tired of hearing it. It's actually a very strong argument. "quasi-public", "maybe Twitch isn't a great example", "maybe not legally... yet", "some forthcoming legal doctrine"-- these are weak qualifiers. What are you proposing, exactly? How many users is a service allowed to have before their content moderation policies are nationalized?
Somewhat ironically, whatever you're proposing would almost certainly require weakening the first amendment. Personally, I'd prefer a strong first amendment (which is concrete and actually means something) to the nebulous "spirit of the 1A".
> whatever you're proposing would almost certainly require weakening the first amendment
I assume you're referring to the 1A rights of the corporation, e.g. Twitter's right to decline to publish content it disagrees with. I agree with you there. It's a complex problem. My complaint is more with the argument that only the government can restrict free speech. It's a lazy argument because the reality is far more nuanced than that. For example, consider the fact that AT&T doesn't have the right to drop your phone calls when you start talking about Verizon to the person on the other end of the line.
It's totally fair to talk about the "spirit" of the constitution -- that's why we have the Supreme Court in the first place, to interpret the constitution when its words are not clear. I would also note that the Declaration of Independence doesn't mention any self-evident truth about corporations, but it does describe the rights of the people, and government's role in preserving them. If corporations appear to interfere with those inalienable rights, it's not unreasonable for the government to step in to preserve them.
Your belief that large social media platforms should be more permissive has nothing to do with the First Amendment, which is a limitation on a grant of power to a non-human entity (i.e. the Federal government). The idea that the "spirit" of the 1A constrains the actions of human individuals is somewhat perverse.
Sometimes platforms ban people I think they shouldn't. We agree on that much. I hope user pressure pushes them into more fair and transparent processes. But I strongly believe they should have the right to publish or not publish whatever they want. And on ideological grounds, I strongly oppose any weakening of the First Amendment.
Somewhat ironically, whatever you're proposing would almost certainly require weakening the first amendment. Personally, I'd prefer a strong first amendment (which is concrete and actually means something) to the nebulous "spirit of the 1A".