I think most people who have a 360 will hang onto it so they can play old games. Once the next gen consoles hit the market, you'll begin seeing less and less games for the PS3 and 360. As I understand it, new consoles having backward compatibility is like the difference between a $400 console and a $700 console. I'm sure those numbers aren't exact since no prices have been released yet but that's kind of the idea that's being put forth.
As far as the DRM and used games, yeah that sucks. MS is the only one that I've heard float the idea of being able to play used games for a fee. PS has made no such statement that I'm aware of. So PS may be devoid of used games altogether in this next generation console.
That estimate is pretty true. To make it backwards compatible would cost hundreds of dollars. Those who already own a 360 probably won't chuck the one they own. It would be convenient to have that feature, but honestly, if it means I save a few hundred dollars I'd be happy to forgo the convenience.
I'm not sure what to think of the xbox one right now. I'm a loyal xbox fan, but there are a lot of things about the xbox one that annoy the hell out of me. The first being their attack on used games, the need to have a constant internet connection and constantly powered, the rumors about the kinect being on pretty much 24/7 and the recognition software. I don't want to sound paranoid, and I'm sure some of this is just baseless rumor, but assuming that's all true, it's a bit disconcerting. I don't really want the kinect on 24/7 monitoring me, even when I'm not playing a game. I heard games were going to be serial coded (or something similar to combat piracy) which I guess I understand, but that's going to get annoying fast.
Right now I'm not hugely impressed. Things might change between now and November/December (the most likely time of release), so there's some hope.