That's not how you bring in adoption or sell products. They should have build something amazing using Kinect that everyone would have wanted one and not cared about buying it separately. Then if everyone had one then devs would have come. But they themselves did not have some amazing use for it.
That's naïve. No matter how amazing your launch game is, a large fraction of your install base will just never bother. Maybe they don't have the money, or they're just frugal. Maybe they're skeptical and reluctant to upgrade. Maybe they just don't like whatever genre of game.
It has been tried again and again and again. Optional console accessories and upgrades just never have enough of an install base to attract a critical mass of devs.
Outside of a few specific games that went all-in, I remember the Wii & Wii U game ecosystem as largely ignoring the wiimote and the gamepad in favor of standard controls.
Going further back in time, I remember the GameCube/n64 and gameboy game-relevant accessories as largely being limited to a few games each, and always as an optional bonus feature that never really mattered that much
