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?

I think this is a great question, and the answer is probably more nuanced and personal, even per use case.

I use fish because of its simplicity, period. And it had all the features I wanted from zsh built-in, with zero or simpler (actually understandable) configuration.

I don't help anyone with bash or write or share scripts, so I don't have that issue where I need to juggle multiple shells, which is lucky for me perhaps. 😌

But those other steps you mentioned probably are a bit more niche, let's say. I think nushell is more tailored to people using the shell to process a lot of data. Digging through logs, perhaps. Debugging systems, or systems management/maintenance. Then it's really handy to be able to process output as data tables, with proper sorting capabilities etc. Nushell is really powerful for that. I actually have it installed because it's already useful sometimes but I barely know any of its specific commands. 😅

Anyway, the choice of shell really is very personal. For me, fish and bash share the same purpose of being used both interactively and as general-purpose scripting languages. And since fish is better in my opinion I chose it. But if I had other use cases, maybe a different job with different requirements, I would make nushell my default. 😁