Both United and Jetblue have extra legroom economy sections available for purchase by anyone well in advance.

I think this shows that the original article is incorrect in its assumption that given more product differentiation people will chose an alternate airline. If more people were willing to pay for extra legroom then more airlines would offer these "economy plus" sections.

For me, I'll stick to flying one airline, keeping my elite status which almost always guarantees me an upgrade to first class.