One misconception that people have is that the next Xbox won't be called "Xbox 720".
Xbox 2 is called "Xbox 360". Compared to Xbox 1, which is simply called "Xbox", Xbox 2 is 360 times more than Xbox 1.
Now, it only make sense that Xbox 3 is also 360 times more than Xbox 2.
And that makes "Xbox 129600" for the next Xbox, namely Xbox 3.