Way too many 26" tires/wheels out there for it to be an endangered species anytime soon... I think you are safe for at least the next ten years if not double that.

I do think that 650b will continue to grow in market share and displace 26" to some extent, especially with the growing popularity of disc brakes and the ability to swap out wider 650b wheels with 700c wheels while maintaining tire circumference on a single bike. That's a big advantage for those of us who like to have multiples pairs of wheels for the same bike. I can only speak for myself, but I'll only be buying 700c or 650b based bikes for now. Not that I would hesitate to buy a nice bike at the right price just because it has a 26" wheel, it just wouldn't be my top choice.