Seeing as Windows mobile 6 was released in 2007 and it's now 2012 I would say that Windows phone 7 might be supported for 5 years as well, after all we'll be a few versions ahead by then and it's unlikely that people will still be using handsets that were released with that version.
I don't know anyone who regularly uses a smartphone today that was made in 2007, do you?
Software platforms and versions move on. You don't continue developing every platform, they need to move the dev teams that were working on platforms based on 2007 architecture up to newer platforms, it's a rolling movement.
Companies must discontinue old software, this will always happen. As for Windows phone 7, it's probably got a year or so before it becomes Windows phone 8 and then 7 might stop being supported in about 5 years.
