These are estimates given by the companies. 10 million units sold by 2007? Sony will be lucky to sell 10 million by 2009. They simply can't drop the system price for 4 years. That will cost them the wars. Sony's sales have halted at 1.2 million units sold. Europe isn't going to extend those sales much further than that.
The better question is how many people globally will pay $600(Even at $500 you count tax, cost of a single game you still get over $600) for a game system?