3. would have to be a new mechanism for variadic functions, that would have to be distinguished in header files from the old mechanism with which it is incompat

3. would have to be a new mechanism for variadic functions, that would have to be distinguished in header files from the old mechanism with which it is incompatible. So this proposal would imply some new keyword or syntax. I am not in the committee, but I don't think this is going to happen. The improvement is way too incremental to force a new syntax. (The committee is fine with incremental improvements, but new syntax need to have strong motivation behind it, much stronger than this.)