Why do I get the feeling that if 650b was the reigning standard, a handful of people would be arguing for the superiority of 700c?
You're almost certainly right. But in all seriousness, I think 700c is a great diameter that works perfectly with the tires that the huge majority want to use. It's only when you get into big puffball tires that the problems start to crop up with 700c, and the basic idea with 650b is to use a smaller rim and a taller tire to end up with a combination of "700x23" outer diameter and a whole lot more air volume. And again, if the manufacturers wanted to invest in some 26" 30-40mm lightweight tires, then the only thing 650b would have is the cool French cyclotouring cachet, which may or may not be of any value to the individual.