I would agree, because NORAD already protects Canadian sovereign airspace, whatever the status of the Northwest Passage. I would say, though, that there was the view at the time that it would be in U.S. national security interest to regard it as internal Canadian waters, thereby providing greater rationale for Canada to know what is going on there, and as internal waters, you wouldn't have the status of transit passage in the area.
As Dr. Charron pointed out, there are reverberations to other international straits. The Canadian position is not supported by other allies, but strictly speaking, from a defence standpoint, I think it would be in the U.S. interest to recognize it as internal Canadian waters.