Except then you go to share something with others and have to convince people to install your esoteric shell first. It’s the same reason why I quickly abandoned fish - zsh with some fish niceties (auto-complete, etc.) gets me most of the way there, and in the odd case where I’m writing a script and use something zsh-specific, it’s not that hard to convert it to bash if necessary.
as someone else mentioned, the shell you use for interactive work and the language you use for scripting don't have to be the same thing. I use fish as my primary interactive shell( on linux) - but scripts for work stuff will still be bash. That said - nushell is now my primary shell for anything windows related. And I'm aiming to do any minor automation on windows in nu rather than .bat files.
One has to be in a very specific mood to adopt a completely different tool as fundamental as a shell. The required investment and commitment is non-negligible. Even if you can convince people of the advantages, quality itself cannot get them magically over the hump in a day.
You have nushell "the language" and nushell "the shell". You can still use bash, fish, zsh for your day to day shell usage. They are proven, rock-solid and super stable. However, for the kinds of tasks that nushell excels -- gluing together programs, doing simple analysis of the output of text oriented programs etc. nushell can be a viable choice. Here I'm emphasing nushell "the language".
Every language was a niche language once when the user base was almost zero. So I guess it depends on your personality and organization -- are you an early adopter ? Or do you like waiting until the tool/language has established itself ?
I would argue that nushell lets you write powerful scripts like bash. But it can be better than bash because it offers powerful constructs, patterns and primitives. I think nushell is here to stay.
