Er, let's look at that shall we:
Well they haven't really. Moving a "Metro" Silverlight project to WinRT requires very little effort at all and can easily be absorbed into an upgrade release schedule - it's not like you have to do a major re-write or change the language or anything. And from an end-user perspective, it doesn't matter as long as everything still runs as it did before. Some apps won't make the best use of my phone, but heck that's true of some of the original WP7 apps that haven't been updated to enable Mango specific features too.
Three posts in a row, both of mine explicitly mentioning WP7 based on the comment vesuvius made on not moving to WP7 development until WP8 comes out in case things change. And I'm obscuring the truth?
I'm part of the MTV generation and even my attention span isn't quite that short!