Similarly, Apple has resolved the increasingly-aged stock Bash install, eschewing Bash's subsequent GPLv3 licencing, by making zsh the default login shell.
I'd like to see zsh also adopt GPLv3 to call Apple's bluff.
dash isn't a fully-featured interactive shell, and would be a major step backward from bash. At least with zsh, Apple are moving forward on a feature basis. (And I say this as someone who's still stubbornly set on bash over that newfangled zsh nonsense ;-)
I'm less familiar with fish, but based on a very fuzzy awareness, it's at least fully-featured.
I do encounter dash on a few systems. It's the default shell on my OpenWRT networking kit, for example. I've installed bash where those systems have enough storage to accommodate it.
I'd like to see zsh also adopt GPLv3 to call Apple's bluff.