That would be devastating at this point in the game. Spring ball just getting started, portal guys all committed and started at their new schools. There's still the late portal I believe, but it wouldn't leave much time to prepare for the football season.
IMO, the best move for Stanford at this time of year would be to promote from within on a 1 year interim sort of deal, then use it as an audition for that person while also looking for their permanent hire if the interim doesn't look like it's the solution.
Just hypothetically, if Mullen did leave, how many of the current UNLV players would be capable of qualifying to go to Stanford? I honestly have no idea, but unless they make special considerations for athletes, it seems like the talent pool they're targeting would be much different based on the academic standards.