That chart is deceptive. For one, it doesn't look accurate. It lists the PS2 as selling less than 70 million and the PS selling less than 20 million unless something is wrong with my browser. The PS2 has sold almost 120 million and the PS 100 million. Also, the PS2 sales would be slower at first because it was at first only released in Japan and was released in the U.S. further away from the holiday season. Predicting the sales of the PS3 based on the PS2 would work a lot better if they both launched at the same time in the same countries.
This chart is a little more revealing: http://vgchartz.com/hwcomps.php?cons1=PS2®1=All&cons2=PS3®2=All&cons3=PS2®3=All&align=1&weekly=1
It shows the PS2 sales going way up after the holiday season while the PS3 sales plummeted.
From what I'm getting from message boards the PS2 sold 10.61 million in its first year, the PS3 will not sell even close to that.
As for that post in the topic, there are quite a few things wrong with it. For one, the PS2 launched for $300 not $375. $375 would have really hurt its sales, although not nearly as much as if it were released for $600
Another thing, the Dreamcast really left the race because SEGA lost so much money. It sold pretty well and had a very good collection of games. Also, even though it was cheaper, the PS2 was still very affordable. The Sony brand name was enough to give it huge sales regardless of the price. As for the Game Cube, it was only $100 cheaper and pretty soon the price of the PS2 went down to $200 so it became pretty trivial. It wasn't a case of a $250 system versus a $600 one. Basically, I think the predictions of most of the users on VGchartz are going to be more accurate than this guy's. At this point it looks pretty obvious as to what is going to happen this generation.