The limitation against T.default can be spun, to many users, as a common sense error check, against uninitialized variables. To more curious users, it can be spun as a conscious reservation for efficient implementations.
This is justifiable for (most) ref types, but what about numeric types? I can imagine many potentially-expensive computations that could reasonably sum to zero. (And as you point out, the smaller the range, the more silly this gets, ending with boolean.) So if we're going to have this restriction, it should be restricted to class types, and the recommended workaround be some sort of Optional or box. And given that not all values will be null-default, it rules out any value types for which the zero answer is a reasonable answer. I think this edge is sharper than it looks.
