IMO, nushell isn't really an alternative to bash/zsh, unless it has backwards compatibility with bash/zsh commands and scripts. I would say it's more of an alternative to PowerShell, given how similarly it behaves to it. It's for people who like how PowerShell works, but want a shell that feels a bit more homely on a *nix system, and don't want to be involved in the Microsoft ecosystem (because PowerShell on Linux is really just a management interface for Microsoft's cloud offerings).