I remember reports from 2005 that said that Microsoft lost about $125 per Xbox 360 unit sold. They made their cash through XBL and payments from developers. With that as context, Microsoft's move to make you buy all new content isn't that surprising.
I think I played my old Xbox games for about 3 months when I got an Xbox 360. My nephews will get my old Xbox 360 games when I finally upgrade. It would be interesting if the XBLA games now function like apps, in so much that they current versions won't be compatible but if developers 'update them', you can re download them etc