I'm not sure I understand this. Those shells only have 90% of bash functionality, but are somehow more portable?

I get the POSIX argument, but I'm not sure I agree. In my experience at least, POSIX is less supported than bash.

I'm pretty confident that any random terminal I have to ssh into will have a bash shell. I ssh into a lot of terminals.

I know where you're coming from, it just doesn't chime with my personal experience.