I don't think backwards compatiablity matters. An xbox is a great system and it's only $150. You could pick halo and halo 2 up for another $50. If someone was a halo whore they'd already own the system and the games.
Notwithstanding, but as others pointed out, microsoft will continue to whore out it's existing franchises. I believe that halo 3, fable 2, and morrowind have already been confirmed for the xbox 360. Who wants to play halo 1 + 2 when halo 3 is out and optimized for the new system?
Lastly an example. Look at the ds. It's backwards campatible with the gba. How many people that own a gba went out and bought the ds? The gba game library is great while the ds game library sucks. But the people that have the gba already have the gba games, they want a good reason to buy the ds. Which the ds has largely failed at in the game arena.
I actually think that the xbox 360 will be very successful. If it launches worldwide by the end of the year, it could have a 3M headstart on Sony and Nintendo. Even if the 360 is the least powerful, it will just become the standard. Do the game work on the 360 and port it over to Sony/Nintendo. As stated xbox 360 has successful franchises which is something that xbox 1 didn't have. And sony won't have thier head start. Honestly, if ps2 and xbox had launched at the same time, who thinks ps2 still would of owned? PS2 only had the games, and that took them a year to get thier games to the market. The xbox was a much better machine and microsoft did a great job of getting several titles on thier system fairly fast.
Side rant. I think the xbox 360 will do more damage to the psp than the ds did to the psp. Here's the thing. The psp is the new cool hip thing right now. But people are largely buying it to use as another home console saying psp is almost ps2. Well when xbox 360 comes, it will obviously own the psp. And a price I saw was $300. Seriously who would want to plop down $250 on a psp when they could enter the next generation of gaming for another $50. This will cool psp sales because the psp will be forced to compete with ds for the handheld market and $250 +$50 games+memory sticks is too expensive for the handheld market.