Using zsh will not fix the fact that other, non-shell POSIX utilities will not suddenly have useful GNU extensions.
Also, zsh is not installed by default on most distros.
Also, zsh is not installed by default on most distros.
Why they went with Bash 2 as the defualt shell is beyond me. I always switched to and used Zsh which had a more recent version. Now I'm also using it on Linux and FreeBSD, because I want a consistent shell.
The reason was the same as here: bash moved to GPL v3.