What is the difference between GPL2 and GPL3 that makes this impossible from a legal POV?
Where exactly the border lies between and not linking is a grey area, legally, that hasn't become clearer with GPL3.
It wouldn't surprise me to see bash go from the standard install, too. Apple is slowly removing all GPL code from its OS.
I use ZSH as my main shell on OS X. I find it superior to bash in almost every way (especially with oh-my-zsh) except in ubiquity. Even so, I have no problem using bash if that's all that's available, the two shells are fairly compatible.
Tivoization was another issue, especially with Apple's tendency to lock down their platform.