the massive push of new wheel sizes comes down to one very simple fact, that is that it is much harder to sell a person something they already have.
forks have gotten about as good as they can get.
better rear shocks have made rear suspension work well no matter what design it is.
brakes can send you into orbit with 1 fingers worth of pressure etc.
the big boys needed a new selling point so they started the whole 29er thing, a good few million $ later it caught on. suddenly everyone (americans) is selling their perfectly good 26″ bikes and buying 29ers and the marketing men are overjoyed. move on a few years and the same marketing, sorry R&D department decide they need a new angle and suddenly 650b is the third coming, suddenly craigs list is awash with the old hat 29ers and any american seen on a trail not riding a 650b is scoffed at by his peers.
at the end of the day the fittest rider will be the fasted to the top of the mountain regardless of wheel size.
and the most skilled rider will be the fastest to the bottom regardless of wheel size.