Why? They still have to support WP7 for at least a year for existing phones that are _still_ being sold.
And WP7 and WP8 are completely different OSes, WP7 uses a WinCE kernel and WP8 uses a modified WinNT kernel. So they have still spent two years adapting OS just to abandon it later.
Both OSes share some level of API/ABI compatibility because apps for WP7 are written exclusively in .NET (with native API only available to Microsoft and a small clique of partners). But that was also true for the plan to use QT to make development possible for Symbian and Meego. And Meego/Harmattan was planned to be binary backward compatible with later Meego.
So the situation would have been EXACTLY the same.