I say they sell more copies of Windows < next version >, because they promise it'll be better than Windows < current version >, so Joe Average has a reason to upgrade. They will sell a whole lot of whatever AV tool they offer with Vista, simply because they kept it out of the OS.
I guess you don't know the software industry. Microsoft does NOT sell AV software. So they can't make money on it. As far as making the OS better to sell it...well duh...of course they got to make it better. Just like OS X and its many flavors. You can't make an OS with everything you want in it and still release it to market. You have to draw a line somewhere.