I run 27" wheels I will admit it is getting a bit tough to find nice tires, but far from impossible. I figure I will keep them until something goes horribly wrong with one (not likely), then I will upgrade.

A bike is a machine all to itself. It isn't like a computer. You won't need to put new software on it or make sure it has compatible ports some time in the future. Use the parts that make sense for what you want to ride, the only obsolescence is in your head. Are there any cars in the world you would consider obsolete?

As an extra side note: People keep talking about 26" wheels *maybe* going, but my impression has always been that they are the most common and most easily found wheel/tire. It will be a while.