For whom? The least volatile asset relative to dollars are dollars. The moment you start building a money-market fund you have novel risks, from credit to rates and FX. (And, in the extreme, legal.)

For people outside of the US.

They’re better off with their own currency or an international currency (currently, Euros or dollars). Almost never a basket. (The IMF has SDRs. And various trade-weighted baskets are constructed for metric purposes. They aren’t practical instruments.)