There is alot of guessing and pure speculation here which people will not even take into consideration for a valid argument at this time, and they are right not to.
But one thing is certain; even if the N6 is just an HD wii, it is still starting the next generation of consoles. Which means the 720 will not have the one true advantage which it had this gen.. the early headstart. That is where the OP is accurate.
But Microsoft knows what they are doing. They will go one of 2 directions with the nextbox. They will either become the leading system for casuals through Kinect 2.0 or whatever, or they will buy certain developers. One or two huge purchases need to be made to create unique exclusives for Microsoft next gen.
If this is the case one giant purchase of Activision or EA would really be something which would solidify Microsofts first party lineup alongside the competition.