The power of backwards compatibility is absolutely KEY for casual gamers
What?
Casual gamers need a couple of 'killer apps' and some time wasters and maybe netflix. They will probably not play them that much and they certainly don't look to dust off five year old games. A new improved kinect sports would probably pull in more casuals that the entire back catalog. The opportunity to play some game they already own and didn't finish is probably not very key for casuals.