Anything whose copyright is owned by the FSF and was GPL2 is now GPL3.

Yup. Only option would be to fork from the last GPL2 release and evolve it as GPL2.

More likely they'll just switch to a new default shell. Perhaps that will be zsh, or perhaps they'll just go back to tcsh.

Or perhaps Terminal will no longer be installed by default by the time they complete their purge.

Or use a different shell, like ksh or zsh.

Or move on to another shell. Such as zsh, or tcsh, csh or countless others that exists.

Is there any reason not to make zsh the default? Better license, better shell.

There really isn't. I hope they do that.