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.

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.

Even macOS runs with GNU Bash.

llvm-libc is in the planning stages.