I do think that XB1 will be MS's last console.
But not because of poor sales, or anything like that. I think it'll be their last console because of convergence - that is, consoles are already very similar to PC, and software platforms on PC are strengthening. I expect that MS's next "console" will actually be Xbox Live on the PC, perhaps formulated into an "Xbox Live Windows" - an OS dedicated to gaming.
Hardware has never really been MS's forte. There's a reason why there's no such thing as the "Microsoft PC" - no first-party Windows hardware. MS only got into the gaming hardware business to fight off Sony, who were stealing all of MS's PC gaming. This new generation, PC is going to be running alongside XB1 most of the way, I think, with games made for XB1 pretty much already being ready to put on Windows without much further modification. And with that, MS's motivation to be in the gaming hardware space fades - PC gaming is set to become pretty much the same as "traditional" console gaming (that is, not counting the Wii U, which is taking a different tack).
Oh, and there's no way that the 360 will overtake the Wii. At this point, it's not even obvious that the PS3 will (for all the pointing at PS2 that people do, the fact is that the PS3 is no PS2 in terms of sales, and there's no particular reason to expect that, just because it's a Sony system, it'll therefore follow in the PS2's footsteps in terms of sales; the PSP certainly hasn't - the PS2 sold over 13 million units in 2007-2008 financial year, the PSP barely broke 4 million in 2012).







