1. Why is it "invalid"? There's no "not accepted", especially for such a specialized topic. It is used by a few already. As asked there, JFK does have concourse A and B in Terminal 4. That remains unsolved. 
2,3. I have already considered backward compatibility. The presence of  `building=` is on those 2  `aeroway=concourse` should be enough. There is no necessity to enforce a one-to-one correspondence between `=terminal` and `building=` . For example, if Sky Bridge is considered as a `building:part=bridge` , can main T1 and NSC be 1 `building=` ? The definition of a `building=` may vary. The main problem here is both MFC and NSC are named "concourse", and marked as T1. That should be consistent with the 5 concourses in main T1. 
4. There is no such thing as you must use  "approved" features https://wiki.openstreetmap.org/wiki/Any_tags_you_like 
5. You know you can discuss on my changeset first? Saying "fix incorrect" without checking doesn't help. You say "and it seems to make no sense to me personally. " , that's why " This doesn't automatically mean you should remove them. They may have been added for a specific purpose. If you think they might be junk then try to contact the author. ". 