I agree with you that Fish is in language design choices and the default features. There is no denying in that and is the reason why I used Fish as my main interpreter for weeks. But that would not change the fact that I read and write scripts, either for me, to share or to help others, let alone the legacy stuff. Bash is the standard.
So scripts would stay in Bash for me, and only the interactive interpreter was Fish. And that was a problem for me. Because Fish and Bash are similar, but they are different enough that I got always confused which way was to do and write scripts. Especially because Bash had some quirks (yes its bad, not denying it), and Fish didn't have them. I thought that I would get used to, but it was always confusing. I rather have a language that is completely different, not similar but different.
So, if I was using a different language that is no longer compatible with POSIX or Bash, then why would I use Fish instead any other language? Why not Xonsh (Python) or Nushell in example? Because that's the category I am looking Fish at, not to replace Bash.