The only conceivable reason for attempting the "pre-emptive strike" strategy of console marketing is when a current generation platform is not able to maintain profitability, whether that be due to a continued loss of market share, an inability to streamline hardware production (both reducing production costs and increasing production) or the most basic being lack of compelling soft titles.
These have been the reasons for virtually every premature demise of any console and currently, none of these reasons are an issue for any of the three current consoles. There's a market established for all of them, and they are all currently profitable and growing in sales over the previous year if not in market share (which is arguably the the least significant bit of data when taken solely by itself without examining the underlying fundamentals).
The only reason the Xbox was prematurely kicked was because it was losing money well into its life cycle. MS didn't even bother to wait and see how successful the 360 would be before unceremoniously dropping its predecessor. It's a safe guess to say that the Xbox would probably have stayed in production if it was generating enough revenue to post a profit like the PS2. It is horse sense to cut a product that is still laying golden eggs and this was clearly not the case for the Xbox.
The 360 is not in the same position this generation; it's profitable.
A new control scheme does not require an entirely new console (GC - Wii) so long as the core unit itself is still viable. Another 360 SKU is conceivable with a motion control remote and a pack in tech demo to showcase what the new peripheral adds, but a new controller alone, is not a reason to start again with a clean sheet of paper unless virtually every major game being designed moving forward is done so around a new control scheme.
As of right now, that's about the biggest thing MS could add to the Xbox, with other corrections/improvements like mandatory HDD, built in WiFi and possibly HD media format for games (and more RAM and VRAM), which would require the introduction of an entirely new platform or else it would mean having to publish games on both HD media and DVD9, which is extremely unlikely and logistically messy.
If MS added an HD media drive for the purpose of publishing games on HD media, then you have a new console, which would then take full advantage of advances in CPU and GPU development.







