The problem is for a name that's stupid. Basically 360 means revolution, what does 720 mean? Nothing. By that logic should the PS4 be called the PS6 instead? PS3+PS3=PS6. Windows 8 should have been Windows 14 I guess. It was an idiotic idea and it's not going to happen, time to let it go.
To be fair, nothing makes sense for the 360 successor name until Microsoft names it themselves. I fail to see how the name "720" is any worse than the other code names Microsoft has come up with over the years (or even actual release names for that matter). At least from a gamer standpoint of someone who might have enjoyed the hell out of Tony Hawk games growing up, 720 still makes sense being another full rotation. If you want to get all mathy about it, chances are you're thinking way too hard on something that's completely irrelevant anyway.
We could have called it Delta after all.