The whole size thing is just another example of marketing push. The big brands get together and decide on what new specifications and standards they can come up with to make your current bike obsolete faster so you buy newer sooner.
It’s no more than that.
The reason 650b seems so popular all of a sudden is just a fait accompli. The industry want’s change, they know we will naturally resist it, so they hit a ridiculous bar with 29-ers. Not many people fall for it, but when the 650b is rolled out as the new messiah everyone is all over them because they aren’t 29’s but so much more like what we already know. Of course no one stops to think “then f_ck it, i’ll keep what I already have”.
Do the math, draw the diagrams a 29er is 8mm better at rolling over vertical objects than a 26″ wheel. But at the same time that same wheel with the same built rim and tyre will be over 200 grams heavier and of course neither as stiff or as durable.
So if you like having to accelerate greater mass more often during every ride, go for it, knock yourself out buy some 29’s. If not stick to your 26″ and protect your investment in the bike you already own.