Why would they have to delay the release for that? The 360 nowadays is nothing like the device it was when it launched. If they did one thing right this hardware generation it was constantly updating the device with new services. If a new XNA is imminent, all they'd have to do is push an update to the 720.
Microsoft has a track record for delivering late to market, "also-ran" products. IMO they could help save the Windows franchise and advance in markets for the Xbox if they get it right for both consumers and developers right out of the gate rather than years down the road.