My observation is merely that every additive change breaks the ability of new code to work with old compilers, and that it would be really, really weird to have

My observation is merely that every additive change breaks the ability of new code to work with old compilers, and that it would be really, really weird to have a system in which you bump a major version because old code continues to work, but bump a minor when it would break or something. If you "aren't proposing" that, then all you seem to be saying is "I think sometimes major versions should signal that they break compatibility with old code and sometimes they should signal that they maintain compatibility with old code because of some never-promised loss of reverse compatibility with new code on old compilers" which ... sounds uninteresting, unreliable, and frankly terrible from the standpoint of anyone who wants automated tooling to be able to make decisions based on signals in the version numbers (which, while not 100% reliable, is still a huge win over the old days of "versions can just be whatever, man" imo)