Agreed, they could... they can, and they have... with handheld consoles. Just look at the various incarnations of the Nintendo GameBoy Advance, DS and 3DS. Same hardware packed into different shells each time with a few small additions/changes that don't really affect game development. Nintendo has been able to get people to buy the updated versions even though they own the previous versions.
With consoles, there are refreshes as well... the "slim" versions of consoles are released, however this ends up being more of an effort to drive the manufacturing cost of the console down rather than getting people who already own the console to buy it yet again like Nintendo has been able to do with its handhelds.
Speaking of which, this is also one of the secrets behind Nintendo's total domination over the handheld market... handheld refreshes getting people to buy multiple versions of the same machine, it's brilliant.
As for the 180 of Kinect on Xbox One. Awesome news about us being able to unplug it and have the console still working, but why do I have to pay for a device I'll never use? I'd rather spend 100USD less for an SKU that doesn't have the Kinect in it. This is a reason why I'll probably end up buying an XBO long after I get a PS4.