IMHO it's basically irrelevant if you would need CPP or could do it in plain Haskell, the main point is: It is an incompatible, breaking change which must be acted upon by maintainers. An ongoing stream of such tiny changes results in a non-trivial amount of work if you want to keep your projects up-to-date.

A much better approach is to bundle such changes and discuss them, come up with a good migration story, implement things and release them, all under a common theme. In our case, it would be something like "use NonEmpty in base and some central packages wherever it makes sense", *if* that is what people want.

I'm basically neutral about the proposed change itself, but I'm -1 if it is considered in isolation.