A company with significant coercive power (through sticky market effects) is much better thought of as a governmentesque censoring entity rather than some mere group of individuals exercising their individual speech. What you're describing is not a virtue of the first amendment, but rather a shortcoming of its implementation and a subsequent failure to properly regulate corporations/LLCs/etc. OP is right - people's ready embrace of corpo-authoritarianism when it lined up with their social mores set the stage for where we are at now. That comic has always been a low point of Munroe's.

Ask yourself who is more legitimized to take such enforcement actions: a handful of privately owned corporations acting in concert or a democratically elected government?

No, the "first amendment only applies to government" is cope of the highest degree, it was always a cope.

You accepted that an oligopoly could dictate what you could do with devices you owned because it suited your preferences and now that you are on the other side of the sword you squirm.

The correct answer was neither all along.

From the first part of your comment, it directly follows that neither Apple nor Google has a required monopoly on the important medium.

IMO, we need to stop thinking in this broken paradigm of "-opolies" (with its loaded requirement to define what constitutes a given "market") and look at the actual coercive power they wield through market stickiness. Apple and Google both wield much coercive power with regards to software running on mobile devices.