Dalek - welcome to the site.
However, you are incredibly, vastly, far, off on your idea of Xbox 360 sales.
The Xbox #1 sold for 4 years, and Microsoft stopped producing it entirely.
The Xbox 360, in 26 months (end of December) will of reached 16m - 9m shy of the Xbox.
There's absolutely no way the X360 will magically drop off of sales to some sort of catastrophic failure. The Xbox 360 has a great lineup of games still slated. If the X360 was going to die, it would of done it well by now. The Wii is out, and has been for a year, and so has the PS3.
Despite the entirety of next-gen being here, with the Wii selling like hotcakes, and having great software, and the PS3 getting significant pricecuts and a few decent games, the Xbox 360 is selling better this year than any year of a Microsoft-branded console, ever.
So why would a system selling much better than it's predicessor somehow fall off, and do worse? Not only this, the Xbox 360 won't be phased out as quickly, nor in the same way as the Xbox. Microsoft has already pleged that the new Xbox will release in 2011-2012 - Giving the Xbox 360 6-7 years of sales. Add in the fact that they *might* not hack production of the console, and go PS2 on everyone (ie, keep selling the prev-gen system alongside the new-gen system), and it could sell even more.
The *least* the Xbox 360 is going to sell is 35m units - well above the Xbox. I still see it doing above 50m units, if it goes that long.
A comparison:
| | Xbox | | Xbox 360 |
| 1st Oct-Mar | 3.08m | | 3.19m |
| 1st Apr-Sept | 2.75m | | 2.82m |
| 2nd Oct-Mar | 2.79m | | 4.93m |
| 2nd Apr-Sept | 2.59m | | 2.60m |
| 3rd Oct-Mar | 3.24m | | |
Notice the Xbox 360 has outshipped the Xbox each quarter it's been on market? Not only is this definitive proof the X360 is selling/shipping better, thus far, since the first week in October, the X360 already has sold 2.0m units. Microsoft will ship no less than 4.5m units this next period (where the Xbox shipped 3.29m units).
Now, can you tell me where in the world can the 360 fail vs. the Xbox? The X360 has already outsold the Xbox LTD in Japan. By the EOY 2008, the X360 will be pretty close, if not already, to the total units shipped vs. the Xbox, a year ahead of schedule.