I've only ever gotten rid of two consoles: my original Xbox and my Gamecube. In the case of the former, it was on its last legs and there was only one game I played regularly on it that was not functional on the 360, so that wasn't too difficult a choice, but with the Gamecube I have always regretted losing it. Between games I just never played and that constant nagging in the back of my mind to check in on my Animal Crossing town, I feel like my GCN would still be very often played in my abode. I've been so tempted to rebuy it, but I just never have $25 to throw away on a used GCN. I mean... that's a sealed MtG event! Or, if I'm lucky, two drafts!
Anywho, unless the next Xbox is fully backwards compatible with the Mass Effect, Halo, Far Cry, XBLA, and Splinter Cell games, I don't see my 360 going anywhere.