Re:
26" will not run out for several economic reasons. We still have spares for 27" wheels even though it stopped being a "standard" around the early 80´s and back then the bike production was way more limited (people had only one or 2 bikes, produced in limited quantities in expensive first world countries)
However, for 26" wheels:
-they were THE tyre for nearly all mtbs between 1980 and 2010, and many thousands afterwards.
-bike mass-production and far-east outsourcing happened on those years, so there are literally thousands of bikes everywhere around the world. If I had to guess, I would say that currently 85% of the world mtbs are 26", another 12% are 29" and the remaining 3% are all other sizes.
It's easy to think 26" is long gone when in the rides with your friends everybody seems to use 29" or 27.5", but remember they are rich guys (World average) on their latest toys. They probably have another 2 or 3 26" bikes at home. In our latest trip in India there was no other size than 26".
Apart what some already mentioned, that 26" might come back again when the industry has already pushed the 28.5", 27" or whatever they can, here there are a couple more proofs of the good health of 26" wheels:
-the Tech Info document from Schwalbe goes on explaining how each tire size is super-wonderful (and more sales for them) and indicates in the end they will continue to produce tires in ALL sizes (pag
http://www.schwalbe.com/en/catalogesfly ... 015_GB.pdf
-screenshot from CNC Bikes, one of the online bike shops I usually buy from. They currently have in stock 19 tyres in 27.5" size, 57 in 26" size, and 97 in 29" size (which includes also the trekking/road 700 size)