Who said you had to migrate away from existing bash code? You can just start writing new scripts in whatever language you want. Especially if it's a shell language and you're communicating via argv/stdin anyway. The continued existence of bash doesn't mean that you can't use anything else.
Did C go away when Ruby/Python/Perl et al came into being? Or did people keep on writing both?
Unless something new come up that provide something significant enough for people to eat that cost and move to do this new version, I don't see Bash going away anytime soon.
It also hasn't changed a lot since it's creation in 1989, for good or worse.