Even if we were to say all phones can be updated, then you get into the--you know, our business model implies layers of supply chain to get it out to them, and there are certain incentives to get it out to them that have to be talked through, so there's nothing to announce there.

-- Terry Myerson

It's understandable that Microsoft would rather continue to support a stable Windows Phone 7 OS for current devices, rather than releasing a potentially unstable upgrade.  Myerson referenced the issues some Apple consumers have had while upgrading from one iOS to another, and felt that this practice was more effective at selling new hardware than improving the user's OS experience.

In trying to defend the decision further, Myerson made one statement that I had to go back and re-read, regarding Google's policy on Android device upgrades:

No Android phones get upgraded, none, ever.

-- Terry Myerson

This statement perplexed me as I previously had an Android device, and remember upgrading it numerous times...but I digress...

It's important to note that ~95% of Windows Phone 7 apps are expected to be compatible with Windows 8.  That's probably more important for developers than end-users, where Microsoft has been lagging behind competitors Apple and Google.  However, Microsoft is confident that their focus needs to be on re-branding Windows Phone:

I think that we need to rebuild Windows, or Windows Phone brand with consumers, and drive demand from there.

-- Terry Myerson