I don’t follow… could you put #!/bin/bash at the top of your scripts, and still use it for scripting, but then use fish as your interactive shell?
Like for instance bulk renaming of files from the prompt, there is no way I could do that quickly in fish's language and having to switch to bash to do that would be unnecessarily slow. Might as well stick to bash.
What I mean to say is that fish would hurt my productivity rather than boost it.
With the customizations I have in my shell, my bash startup times were creeping into the multi-second territory, but I got a better UX and faster shell startup without having to learn new syntax for simple shell loops.
Context: I used bash for about 5 years, zsh for the next 10 before switching to fish.
I honestly think you might misunderstand why a lot of people use fish. I know bash “pretty well”—I’ve written and maintained my fair share of complex bash scripts, and remain pretty comfortable opening a shell in a running docker container or EC2 instance to debug various nonsense. What makes fish attractive to me for use as my login shell is the creature comforts which are very nice to have on my regular workstation, but aren’t really necessary when mucking around in some random environment: syntax highlighting, tab completion/autosuggestion/history search which “just works”, somewhat nicer syntax for writing simple scripts for personal use, saner word splitting, and so on. I guess I can’t really speak for other fish users and am perhaps biased by learned bash first a couple decades ago, but I don’t personally find it especially burdensome to have to remember multiple shell syntaxes/semantics (I already use half a dozen in my day to day work; what’s one more?), and I view it as a reasonable price to pay for a UI I find much more pleasant on whole.