That's all working off the assumption that you change frames, forks, and wheels (or just a complete bike) every damn year. Once I find something I like, I just stick with it. My 2010-2012 generation Enduro for instance.Wall mart will have them. I suspect you'll be able to find decent low end rims that are at least strong for things like snow-bikes and specialty applications like dirt-jump/DS/unicycles (lol), etc. For a long time you'll be able to get rims and parts though, as the molds and mills are already in place and will continue to make as much money as possible before switching. I don't expect any crazy prices though, as it's a balance between not making the buyer give up and move to another wheel size. It'll just eventually phase out. The only guys complaining about not being able to use tri-align brakes or square taper bottom brackets don't want them to win an XC race or DH race and you can still find some of that stuff out there. This is really much ado about nothing, even if they are going to phase it out. Look at the 1.5" steerer manitous. Completely worthless now due to tapered frames. Who cares? Those died a long time ago and for a while you could get plenty of 1.5 all mountain/freeride frames. By now no one cares anymore. Same thing will happen here. By the time you are "forced" to change, you won't even know it because your bike will have cracked or exploded.
I must say that a rim/tire manufacturer would be absolutely suicidal to just phase out 26" production altogether. There will ALWAYS be significant demand, as long as they are made, and as you stated, upfront tooling and production costs have already been dealt with. No reason NOT to make the most of that for as long as possible, besides blatant marketing crap.
But, it's the bike industry. So, I take the logical, secure, financially sensible long term plan, take the opposite, and expect that.