Back when I last bought a bike, it was simple – there was 26″ aka 559mm for MTB (and 700C for road). I’m now thinking of buying another MTB, which will be expensive and I’d expect to use for at least a decade, hopefully 2. Am I going to find decent tyres hard to find at some point in the future if I buy the wrong size? I’m tempted by 29″ aka 622mm because I’m big and will be riding mostly fairly tame stuff (more MTBO and natural routes than trail centres).

Maxxis just released their new rubber in 26″, stans some new rims so I’d say which ever you choose it’s not going to be much of a problem.
29 or 650 will be around for a while.

26″ never died. It was perhaps a little poorly for a while but it’s too wide spread and too many people refuse to drink the latest Kool-Aid for tyre and rim manufacturers to not support it. 1 1/8″ straight steerers not so much though 🙁

Looks like 26″ is being kept alive and well by cycle speedway

http://singletrackworld.com/forum/topic/cycle-speedway-8

In answer to the OP my plan is to not buy another MTB. I’ve got a 26″ straight steerer Inbred which is fine for all the types of riding I do, it’s steel so should (!) last and if I upgrade anything I’ll prob have to upgrade everything, so I can’t be arsed. Humbug!

Yes TBH I’m not so worried about 26″, but the new bike is available in 29″ or 26″ and I reckon 29″ will suit our riding better. Bit of a punt either way but that’s inevitable for a tandem.

tbh buying anything that has consumable parts and expecting the ‘standards’ to be around for a decade or two is basically ignoring what has occurred over the last 50 years or so. Stuff changes, accept it and just buy what you want to ride.