No software company is--or should be--obliged to support a product until the end of time.
If MS isn't going to issue patches, there ought to be some way for someone else to be contracted to. Buildings are essentially open source so that is easy to do. Software isn't necessarily.
My point was that most buildings aren't kept around forever either, as you were implying.
How?
Microsoft already issued a patch( not that they didn't support XP a long time over its lifespan ), it is called Windows 7, Apple does this as well, with their yearly OS releases, even though in a more clever way.