That seems redundant to have two building tags. It should be either aeroway or building when the associated airfield is shutdown. The are plenty of was of denoting a hangar is for military use such access, landuse or even owner/operator fields.
I don’t think that anyone’s proposing that? A “building” tag describes what sort of building it is, not what it is used for. If something was built as an aircraft hangar then that is what the “building” tag should be, regardless of what it is currently used for.
“aeroway=hangar” is a description of the current use. Either tag can be valid without the other, though given the size of aircraft most “aeroway=hangar” were probably built as “building=hangar” (maybe with a few “building=farm_auxilliary” on smaller farm airstrips).