I’m a cats contributor and I think this would be a mistake to try to put all of cats core into the standard library.
Dependencies are not that costly if a big consideration is given to binary compatibility. The main offering of the standard library in my view is binary compatibility across an entire version of scala. In fact, cats has already done this: version 1.x are backwards compatible and 2.x are still compatible for scala 2.12 (which has more forgiving rules around traits).
I think we should move the other way: a smaller standard library and better tooling for maintaining binary compatibility for libraries. For instance: it would be interesting to have a way to do small binary breaks if we could easily see that virtually no downstream would be broken by that. Or perhaps tasty could offer us much more forgiving binary compatibility. Or of course, we could investigate more source based dependencies where incompatibilities become compile time breaks where they belong rather than runtime breaks. Or we could imagine a linker that runs on binary outputs and finds (or warns) of possible binary incompatibilities statically. Or…
A lot could be done here, So far almost all we have is MiMa, which is really great, but we could do more in this space.