The timing of the launch won't matter much when Microsoft has a bunch of other things to address, but if we ignore these other things for a moment, preparing for a successor rather sooner than later is the logical thing to do. That is, if there's still any ambition to sell a lot of consoles. Although looking at the big picture, the most sensible course of action for Microsoft would be to let this generation fizzle out and go full third party, because if they kept making consoles, they would also keep doing things that contradict each other. Namely that they try to push a console and platform-agnostic services concurrently; this just doesn't work, because it makes the console redundant.
Legend11 correctly predicted that GTA IV will outsell Super Smash Bros. Brawl. I was wrong.