To the extent this is true, it's because other software relies on specific features (and sometimes quirks) of bash and glibc, not because you can't have a workable Unix without them.

Which is to say - ironically, these last remaining dependencies wouldn't be there if open standards were followed more rigorously.

Ah yes, textbook "Embrace, Extend, Extinguish" strategy. Very ironic that GNU now looks like it's following this pattern (obviously unintentionally).

Alpine Linux and many other distros use musl and busybox (non GNU projects, although busybox is GPL).

And I would definitely call them Unix systems, since they run almost all the same software.

Although GCC is still a good argument, since very few distros use Clang by default, I think mostly because of the Linux kernel.

I think ZSH is a valid alternative to Bash.

OS X Catalina even made ZSH the default.

IIRC, not just a alternative, but a full super-set.

No, Zsh is not a full super-set of Bash.

I don’t.

Even macOS runs with GNU Bash.

Was replaced with zsh recently

Apple's Command Line Tools installer installs GNU Bash.

Bash is still available.

Not for long, most likely.

llvm-libc is in the planning stages.