So how much does releasing a console first in a generation relate to sales?

[supermario128]

By that logic the 360 would be leading the current gen of home consoles, the Dreamcast would have sold more than the PS2, the TurboGrafx-16 would have sold more than the NES, and so on. While releasing first gives the product more time to sell, there is no guarantee it will do well. Your premise if illogical.