I imagine if demand gets really low they'll make major changes to the system partway through its life. Sony managed to turn around their initial slump with the PS3.
Get used to always-online gaming though. There's speculation that by the time we reach the Xbox 4, it'll utilise streamed graphics - all the processing hardware will be on the other end of the broadband cable, making the power theoretically unlimited, and potentially making console 'upgrades' a thing of the past.