The primary reason I keep writing is that this, the plain written word, is The Platform to transport ideas in time and space. It has passed the test of time in ways that each of those platforms you mention have not (nor any other medium, e.g. video, etc. at least in our lifetimes).

I think of those platforms more as distribution / syndication mediums, using them for stuff like throwing it out there and reach others (which I somewhat care about because the topics I write about are those I like to exchange opinions about with others, but not obsess in terms of needing validation) and keep some degree of isolation from their policies, relative importance vs others, algorithms, UI/UX choices etc.