The bash that comes with macOS is a fork of the last version distributed under version 2 of the GPL, most likely because Apple doesn’t want to distribute GPLv3 code as part of macOS, and it is ancient. They keep it updated with security fixes but nothing else, so it has gradually become less and less compatible with current bash.

Since zsh has become a popular bash replacement and it isn’t GPLv3, they switched the default shell to that.

How low

T. rex was around for 20 million years or so, I’d say they passed for long enough to be considered successful. Despite the tiny arms.