Fair but I think it almost never costs more to censor than to allow. Humans like to be in spaces that other humans, either implicitly or explicitly, aren't allowed to enter. We'll gladly pay for that. And most humans also don't like labour, including the labour of having to create and maintain such spaces themselves (those that do are outliers). Humans love to outsource. Therefore, for an online business, setting rules drives revenue, and even if it costs more in absolute terms to censor this or that, those costs are offset by the gains in revenue from people willing to outsource the labour of curation.

You are exactly correct. The profitability of a platform is determined entirely by human choice. And one thing that can be counted on is that humans will choose platforms with prejudicial content filtering. A person can already host their own speech through individual effort alone. Similarly, indexing and filtering internet content can already be done through sole individual effort of those who would listen to that speech. In a way, we have perfect free speech on the modern internet already, even without needing to build new infrastructure. You can hardly be said to have a platform in the first place unless you some form of tyrannical filtering, in addition to hosting and sharing user content. My argument is that these services need not be mandatory. In fact, they SHOULD not be mandatory. I believe that given the importance of user choice, in a well designed platform, the tools to achieve cheap and perfect free speech would already be built in by default. I believe that to have a platform that does not at least have the option of allowing users to post or view any content is either putting in extra unnecessary effort in order to artificially block some uses of the platform, or the platform is ALREADY shooting themselves in the foot by not respecting user choice as much as it could be.