I think POSIX compliance of interactive shells is completely irrelevant. POSIX shells are relevant when you're writing scripts that will be distributed, but there's no reason that needs to be done with the same shell you use interactively. I use fish presently and zsh before that for nearly 15 years, but my shell scripts have always started with #!/bin/sh

It's not like fish installs to /bin/sh, nor are /bin/sh scripts executed using fish just because your interactive shell is fish. I guess these are the two misconceptions the common worries are based on?