FWIW, I consider `at` to be similar to some of the things you consider "new capabilities". `fetch` vs `XMLHttpRequest` is fairly analogous to `at` vs `arr[i]`, and similar arguments can be made about ES6 classes over prototypal classes, etc.
> `fetch` vs `XMLHttpRequest` is fairly analogous to `at` vs `arr[i]`
If you consider all syntactic sugar to be equivalent, no matter how minor the change, then anything above a basic Turing machine implementation is fairly analogous. The problem with XHR is that the interface was really bad. The interface for arr[i] is not really bad.
Similarly, sure you can do point-free style w/ ramda's `R.add` but realistically, do you really? The fundamental game changer capability is the language's ability to do math in the first place; anything on top is arguably cherry on the cake. `at` seems uncannily similar in that regard.
Wrt something being in a 3rd party library vs stdlib, I'll generally prefer a batteries included approach in JS because module resolution is complex enough to cause difficult problems to troubleshoot (peerDeps hell, complex symlinking semantics, library duplication in bundles, core.js explosion, package manager specific breakages, etc)