Thanks for elaborating. Yes, it's a common issue to encounter configuration scripts, .profile files, etc. that use POSIX syntax, and it's understandable that people will be unwilling or unable to port them to fish. Usually these are just standalone scripts which can be run directly via /bin/sh, but sometimes you want them to be part of your interactive shell.
https://github.com/fish-shell/fish-shell/issues/522 tracks having a bash compatibility layer - as you can see there's a lot of discussion!