I'm no scientist or anything.

Shouldn't there come a point when the tube is under pressure that the interstitial spaces are too small for air molecules to go through?

Thus they would never go completely flat. The curve would end at some point.

I can imagine a manufacturer would try to perfect their product to balance between the natural properties of the rubber and the amount of stretch at pressure that increases the size of the interstitial spaces.

Anyway, that's my thought experiment for the day.

Karen