People keep calling 29 progression or evolution. It's neither. It's people throwing shit against a wall and seeing what sticks.
The fact the bicycle marketing companies and biking media were teaching us why 29'ers were best for pedaling, but 650b was the one for DH only a couple of years ago is testament to this.
The only defensible position proponents can take on this issue is that it makes it easier for them to roll over square edges. That's it. (And we best not bring mathematics into the equation, would hate to point out that that extra $5000 expenditure could have been offset with a $50 lesson).
Finally, it'd be great if people stopped bringing WCDH into this, because you have absolutely no idea how fast anyone is riding and it has absolutely no impact on your enjoyment of watching the worlds best do what they do. If we made them ride 24" wheels, they'd still slay the fucking course and it'd still be fucking awesome to watch. Mandating tire sizes at their level would only ensure the best riders rose to the top as opposed to the most marketable on the best teams.