Ah, good point. In many cases, you also don't want to install an alternative shell, like if you're dealing with lots of containers and would need to make Fish available in each one.

Although, I will say that more recent versions of Fish are available as a standalone executable, so you don't have to install it in the traditional sense. You can probably transfer it together with your script file in most cases. That's a relatively new/experimental feature of Fish, though, so no idea how well such a workflow goes in practice.