Um, that's pretty much already happened a few times. Windows has been dying and getting replaced with something new on an on-going basis. Microsoft just keeps calling it "Windows" so people accept it as familiar because they're comfortable with the brand name.
It's still Windows. And aside from certain vendors who like to use OS changes as an excuse to force users to upgrade <cough>intuit<cough>, there's little that I can't run on 7 that runs on XP (probably 8 to....Intuit not withstanding).