You are absolutely right that putting all the eggs in one basket is a textbook mistake, and one should diversify communication channels & sources of income.

>nor the right to publish using someone else's equipment

Two legal cases come to mind:

the recent decision that POTUS twitter feed is a public space (https://theconversation.com/federal-judge-rules-trumps-twitt...)

and the 1980 decision in Pruneyard Shopping Center v. Robins where, based on Californian law, private property of the shoping mall was deemed open to political speech (https://en.wikipedia.org/wiki/Pruneyard_Shopping_Center_v._R...).

It's perfectly fine for FB, Twitter, Google & friends to declare themselves editorial organizations rather than open platforms. However it's worth noting Section 230 protections hinge on being a platform.