Is there any risk of Bash ever going away? It seems like it's the de facto shell. I remember considering whether or not I should learn Perl at one point. It didn't even feel like a choice with Bash. Trendy shells seem like they have no choice but to support it too.

I feel like what usually makes me reach for something beyond Bash is really a matter of wanting or needing some dependency that wasn't written in it for whatever reason. Usually this happens right at the point where the script/utility starts to turn into a library/program, so it's trivial to just transpose the control flow into whatever language is required at that point and go from there. This of course raises the type of concern you mentioned about Ruby, but at that point it's hopefully worth the trouble to address.

The only “danger” to bash is if zsh completely overtakes it - and even then scripts will probably be written in bash-compatible zsh.

bash is no longer the default shell on macOS

Bash might be consistent, but what about the programs you're calling out to? Even basic utilities have different options between BSD and GNU Coreutils. Something like git might not have options you're expecting due to differences between versions. Or if you need to download a file using HTTP, you will run into a problem when you run on a machine that has wget when your script was expecting cURL.

And yes, you have these sorts of problems with other languages, but my point here is that Bash doesn't free you from them.