Im not sure the Switch can sell 120m units based on its current tarjectory.
Particularly, is there an audience of 100-120m people who want to mainly play Nintendo's games? Their only consoles to go over 100m relied heavily on appealing outside of their core userbase (wii/DS) or being the only big player in the handheld market (Gameboy).
I'm inclined to think the Switch will end around or below the PS4 with a steep decline in later life.