'Windows 8 will make or break WP7 (or 8). If Microsoft can make it simple to develop an app that works on both WP8 and W8, they'll break the apps = users problem in one step.'
Good point. may I add another?
If Microsoft makes it impossible for WP7 users to upgrade to WP8 they will see a massive backlash from people finding themselves orphaned. At the moment Microsoft is playing its cards close to its chest over the whole upgrade path which isn't terribly encouraging.