The PS3's 4th year hasn't even matched the PS2's 2nd year. It's sold just over 30 million in just over 3 years. It would need to keep up the 10 million/year average for 7 more years. And since it's not the market leader, it won't last 10 years, no matter what Sony says. Just because they'll be supporting it into the next gen doesn't mean customers will be when they have 3 new home consoles to choose from, 2 new handhelds to choose from, and the Wii has a userbase of well over 100 million by then.
But like wholikeswood said, 80 million is possible and would be a damn good success for a 2nd or 3rd place console. It's definitely blowing away the DreamCast, XBox, GameCube, etc.