Piracy was a major factor in PS2 and Wii success. PS4 can't be pirated, so it's not going to appeal to the mass audience that requires piracy to stay in gaming (Some people geniunely can't buy games, others can afford them but think it's a waste of money), so those people will stick with PC where they can always find a way to play a game illegaly, usually even with DRM involved.
In short, PS4 will have a heatlhy life but I believe it's highly unlikely to achieve PS2 sales unless there's a significant change in the market for some society-changing reason... ...Though miracles can happen.







