Plus, let's face it, the PS2 consolde's hardware was excellent, and even rivals what the Wii can do today (not including the Wii's industry-changing Wiimote).
I would predict that by this time next year, as the Xbox360 continues to fall behind in Japan, we'll see the Wii in 1st, PS3 in a solid second, and XBox 360 in 3rd. By year three, as prices drop further, the PS3 will move to 1st, followed by the Wii.
It's a fluid marketplace. By the time a PS4 comes out, the Wii or PS3 software sales will likely still out-sell any PS4 games after the first year.
It's called market saturation.
In fact, the only mention of software sales was this: "From a pure revenue perspective, the biggest system for software sales in 2007 was the PC, if you include revenue generated from online services."
Of course the PS2 is going to have the largest install base of any system still on the market, since it's been out far longer than the others.