Discussions about which wheel size is here to stay are pure speculation at the moment. Anybody who tells you that they know which will win out in the long run is either deluding themselves or has their own agenda. On the plus side, that means that you can just pick whichever one you like and you’ll have just as much chance of being right as anybody else.
I think we can say that spares for all three sizes will be available for the next 5 years or so. Even if manufacturers keep up the 650b push for a few years (which is an assumption) it will take that long to make a dent in the installed base. Not everybody changes bikes every year or two.
I’ve been riding a 26″ bike and a 29er for the past six months and I honestly couldn’t tell you that one was better than the other. They each have their advantages and disadvantages. I do know that I don’t feel any need to a add a third wheel size to the mix. But I also know that if one of my existing two sizes disappears from the market in 5-10 years I wont be all that bothered. I’ll still be able to buy a bike that rides the way I want it to and by then all my 26″ and 29er parts will be worn out anyway.