The principle of common carriers, over which all (lawful) conduct is permitted, including postal, telegraph, and telephone services, as well as equal-time and community-access broadcast services, offer alternate models.
I'm finding the "private platform, no free-speech rights" argument increasingly thin on platforms aggreggating hundreds of millions or billions of eyes and ears, and increasingly inextricably bound to social, commercial, and governmental institutions and activities. At the same time, the wild west alternative has also proved untenable, with strong civil-rights groups including EFF and ACLU increasingly acknowledging and accommodating this reality.
There's a middle way.