> If we make stoi work with string_view then it has advantages over strtoll and the existing stoi(const string&), and can still be faster than strtoll. There's room for something between the lower level, less flexible from_chars behaviour and the current stoi family. 

Ok. Then what exactly is it that std::stoi does differently in terms of behavior that in your view justifies “hey you know what, we need this for this killer feature, and that is worth all the trouble to make it standard and maintaining it in all major libraries”?