Who out there, MWI-adherent or not, seriously thinks that QM is a fundamental rule of nature for everything BUT gravity?
Not sure what you are asking, but Everett and many other MWIers certainly thought/think that "the wave function of the universe" is all one needs to know.
What is something you used to believe, preferably something concrete with direct or implied predictions, that you now know was dead wrong. Was your belief rational given what you knew and could know back then, or was it irrational, and why?
Edit: I feel like some of these are getting a bit glib and political. Please try to explain what false assumptions or biases were underlying your beliefs - be introspective - this is LW after all.