That's weird because the XBox 360 wasn't released with a Halo game. And the game that has sold the most for 360 is Gears of War, and none of these links to lists of top selling games have Halo 3 at the top. As one of my favorite quotes from Perfect Dark go, "You cannot make accusations like that without evidence." While it may be true that Halo 3 made the XBox 360 a profitable product to sell, games like Rock Band and Assassin's Creed and Mass Effect were just around the corner.
http://www.pazsaz.com/top360.html
http://www.gamespot.com/games.html?platform=1029&category=&type=games&mode=top&sort=views&sortdir=asc
http://www.gamerankings.com/itemrankings/default_x360/1029
Besides, every console has its staple franchise. That's like saying "The only reason Nintendo made Wii's is because of Super Mario Galaxy." when Twilight Princess was selling like wildfire when the Wii came out. There are other games you know.
From what I see, Microsoft has enough cash to throw around to make a console whether or not there's a Halo game. And if one were financially skeptical of making a console, Halo 3 would have to be the one and only economic gift from god that outsold all other games forever to make a justifiable "this game is the only reason we're selling XBox's" case. Microsoft is going to make its next console based on a million other factors as opposed to one game.
I thought people had already stopped making stupid assumptions like this when the X-Box 360 came OUT, you know, when it was topping out competition with a year to go till its apparently "only game" came out.
On another note, I really can't wait till the new Metal Gear Solid comes out. It just looks amazing.