In my reading, what Stroustroup is saying is that given other problems in c/c++, that singed sizes are less bad than unsigned but both have clear and significant deficiencies. A new language doesn't have to inherit all of these deficiencies.

No. He says that signed/unsigned arithmetic is a universal problem. And in the context of std::span, using signed arithmetic is the correct choice rather than shoehorning in size_t to make it more cosmetically consistent with the rest of the STL.