> - let us say tomorrow everyone were to consider removing all unsigned variables across c++

We would have to at the same time consider removing all bit operations (shifts, bit_and, bit_or, complement, xor, ...) as they are generally unsafe for signed integers.

That would probably kill a lot of low-level optimisations you were never actively aware of (some tagged versions of SSO, or pointer tagging in general, ...) but relied on for performance.

So, without thinking of further consequences, I think C++ would be worse of with that proposed change.