One final comment.

If writers ignore Snyder or Truby or whomever, isn't there a danger that the storytelling will collapse into a formless mush?

Well, yes, that can happen, but it's not very likely because the writer has already assimilated basic story form (not formula, not invariant paradigms) from watching hundreds or thousands of movies. You have to work hard to avoid incorporating some sort of familiar form (e.g. a three act structure). Developing and harnessing a sense of form is part of mastering the craft (hence your advice to watch movies and read scripts).