You just told the story of Vista vs. OS X, and why OS X is still ahead.
No I told the story of developing applications. How far back is software made for OSX compatible? Does Safari work on System 7? Eventually everybody has to draw that line in the sand, downward compatibility is a lot of additional labor for steadily decreasing return, this is a platform independent problem. The only real advatage open source has on this is that while the guys "in charge" of a product can set a cut off point if there are enough people in the extended community that want the new rev to support some antique OS then they can do it themselves. But even that isn't a real solution, that's a community based work around that assumes there are going to be enough skilled people that care to extend the product's functionality beyond the line originally intended.