All the wheel sizes have a place and have their own benifits/negatives. 650b might not be exactly between 26" and 29" but is enough to make a difference. If it's agreed that 26" is best for DH and 29" is best for XC then surely it stands to reason that 650b is probably around the sweet spot for the type of riding most of us do?
I don't like the fact 650b seems to be being forced on us in a different way to 29ers any more then anyone else but it's all happened before so I struggle to get too over excited about it.
The arguement I hear the most is new wheels, new forks, new frame, waaaaaaaaah. Did the same people make the same noise when tapered steerers and bolt through axels were forced on us? Try buying top end 1 1/8th 9mm QR 140mm+ forks for your wheels that were brand new and current 4 years ago.
I'm not convinced that they do know what wheel size is best. Giant are a prime example having produced 29ers for a fair old while, they've now almost abandoned them for 650b. Both of which aren't necessarily the magic potion to make things faster...
The tapered head tube isn't an issue on frames because you can step them down to accept a standard fork. Depending on your wheels you may be luck with axles....my next bug bear. We had got to a point where most people were happy with QR or 20mm on longer travel forks. Then Fox and Shimano decide we need a middle option. We didn't but it seems to have been adopted and is finding its way on longer travel forks.
No doubt one of the next big things will be ever higher prices but people are buying stuff so I guess companies can charge it. XX1 is a prime example...£200 plus fir a rear mech? No thanks. I'd cry if or when it got bashed on a rock!
I'm far from being anti progress because some stuff is massively better now but I don't remember stuff being forced quite as much as it is now in the 16 years I've been riding but hen again its much more popular than it once was so there's more money to be made.
2015 Stanton Ti Switchback
2006 Specialized Demo 8
2013 Specialized Allez Sport
1999 Santa Cruz Chameleon
1994 Cannnondale Beast of the East (rebuild still in progress)