Hopefully it will give a sustained boost to 360 sales, but history has shown that add-ons don't really improve sales that much (I'm looking at you Sega Megadrive Mega CD AND 32X) for the console in question. Even if it is released as a 360 Natal Unit I don't really see it boosting sales that much. Halo Reach might boost hardware sales for a month.
Off topic, with the constant release of quality software, this generation still feels quite fresh.