Semi-relevant: a PR to add checked equivalents of Sum and Product to core:
I get the original impetus for avoiding having something like num-traits in core, but it seems like they've had a long time to bake out-of-core.
num-traits has been versioned v0.2 for over 4 years. Is there really a reason to be concerned about having these traits in core at this point? It seems like they've had a long time to bake and people are generally happy about the shape of them.
Certainly another round of considering the design is in order, and I would expect a long period of such traits being nightly-only, but it seems like it's achievable to eventually stabilize such traits with the consideration that they're effectively immutable and inextensible.
tl;dr: I'm not sure there's much more research in this problem space which will be beneficial versus shipping something that will make most people happy.
Even if such traits were unstable and nightly-only, it seems like having them could greatly improve the internal implementation of the core numeric types, which presently have heavily macro-laden implementations that, in some cases, could be replaced with traits for expressing common functionality instead.