I honestly believe 26ers are the next big thing. Most of the world rides 26ers. Most of the world are relatively shorter, smaller people. The western recreational cyclist is almost a boutique niche market for bicycles where temporary marketing fads dominate.
29ers are seriously problematic because they require 12 percent more effort to generate the same amount of torque, have higher center of gravity, enormously stretched out wheelbases that hinder maneuverability, weaker and heavier wheels and tires. It's difficult or impossible to design an effective geometry for 29 inch wheels suitable for short-to-normal-height riders. The front end is always going to be too high, the wheelbase is always going to be too long. 27.5 wheels were re-marketed to mitigate these problems while maintaining a viable but faddish business opportunity. 27.5 is the death knell for 29. Not anytime soon but eventually.
700c makes sense with skinny tires and lightweight wheels as on road bikes, but not so much as they become bulky and heavier in mtbs. I think that 26ers will eventually dominate the gravel bike market because there's no significant benefit to the skinny but heavy tubeless tires on 700c gravel bikes. 26er gravel bikes give you a wheel/tire combo that weighs about the same, has about the same size wheel/tire diameter, but gives you a lot more traction at a lot lower pressure.
There's some benefit to larger wheels on a fully rigid mountain bike but the cost/liability is significant enough to favor smaller wheels.
Last edited by Clem von Jones; 09-28-19 at 10:36 AM.