Hey there Dirk, clear and insightful question.
Everything has a limit and we are simply working on extending your limit.
Your proposition that eventually the width will be so narrow that the “aperture will be obstructed” suggests a clamping down of the lip tissue which of course is not the idea. The centre of the lip, however small, can remain responsive. Whilst your idea makes mathematical sense, it doesn’t need to go to that extreme to get in to the altissimo register.
It is possible play G above high C and above with no exertion below the head ie. simply using the air in the mouth cavity. Sure, it will be a very short note and get incrementally softer as you ascend but it can be done.
Working through the 6 Steps of the Singing C Series will extend your limits. Just take it a semitone at a time.