Because Bash changed its license to GPLv3 it is not likely that Apple will update the builtin Bash anytime soon.

That is not really a problem because you just:

$ brew install bash
$ brew install bash-completion@2
$ chsh -s /usr/local/bin/bash
$ sudo bash -c 'echo /usr/local/bin/bash >> /etc/shells'

and now you can use cool things like globstar, coproc, associative array, ...