Complete replacement of a shell?

I am the only user. I use fish.
Is there any way to purge bash from the system?

If you are serious, I guess it is hard to do, since quite many programs rely on bash or are written in bash.
Maybe some distro concentrating on removing bash? Don’t know if such exist.

But why would you like to remove bash?

You can change your default shell to fish easily with chsh.

BSDs generally use tsh csh, ksh etc.

As @manuel said, if you uninstall it there is a big chance you wont be able to run most of scripts, for example sintax used from bash is different from ksh, so you’ll need to rewrite part of the code if you use array, this is just one example.

There is no need to remove it.

Good to know! Thank you.