The Xbox 360 had 8 years on the market before the Xbox One launched and PS3 had 7 years before PS4 launched. Looks like PS4 and Xbox One will have 7 years to themselves before their successors launch this holiday season. I do think PS4 and Xbox One will beat out PS3 and Xbox 360 in the end. But not by a lot. Less than 5 million.
You may be right about less than 5 million, especially if PS4 never gets a price cut. They can definitely lose customers by pricing them out of the market.
If PS4 gets down to at least $269, like the PS3, then I think there will be at least a 10 million difference which is similar to the loss going from Generation 6 to 7. Those 10 million were lost to the Wii, but that market came back to Sony/Microsoft in Generation 8. Instead they might just lose people who can't afford the current price.