You can have other shells that are not bash, true. But you are guaranteed to find a bash shell in any Linux distribution (well, except minimal distros like OpenWRT or Alpine Linux that for saving space use busybox). Let's say that in any server or desktop Linux installation you find bash.

You would probably find also python, but what version? Python3? These days you are probably guaranteed to find them, except that there are old Red Hat servers still in production where is not installed. And Python3, what version? 3.7? Older? What features are safe to use and what are not? Do you have to research to find out? Or you are stuck with using Python 3.2 just to be safe? Well, in the time you wasted thinking about that, you would have finished writing the script in bash.

Agreed. I think bash is a dependency. Even more, it's a dependency I'd want to avoid at all cost.