What would Microsoft do if, due to the economy, individual users forgo upgrading to Windows 7, and decide to keep what they have. Will Microsoft go and say, "XP is old and we won't be supporting it anymore. Also, by the way, VISTA will only be supported for another year."

At that point, Microsoft would alienate users of previous versions of Windows, the users will decide that they need to choose something else, and a large corporation ends up loosing a large chunk of the marketplace.

I would say that Microsoft needs to test the waters prior to dropping support for XP, otherwise people that could decide to upgrade after the ecomony improves would end up getting comfortable with whatever alternate they end up choosing and tell Microsoft that they're not coming back!