They might not be hard to learn, but it's still a considerable mental overhead to switch between writing JS on the frontend and anything but JS on the backend.

They might not be hard to learn, but it's still a considerable mental overhead to switch between writing JS on the frontend and anything but JS on the backend. I frequently mix up the simplest things, like len(list) vs list.length, or how that you can't use obj.property on Python dicts, or list.append() vs array.push(), etc. And then you might have a great tooling library in one ecosystem (like lodash in JS) and try to replicate the things in your backend language. That sort of context-switching simply isn't worth it in my opinion.