Yeah, the PS1 and PS2 had legs due to late launches in a significant amount of countries. PS3 didn't have late launches and neither will the PS4. Sony is also going for profit and until they change my mind with price cuts, I don't think they will surpass 115m-125m sales.
Yeah looking into it further, price cuts no doubt play a huge part in PS1/PS2 legs, they got ridiculously cheap. Here's the last official price cuts I can find for USA.
PS1 - $49 (May 2002)
PS2 - $99 (April 2009)
PS3 - $269 (Super Slim Launch) (September 2012)
It's crazy to think that the PS3's RRP was only dropped to a minimum of $269 and recieved no price cut after the PS4. It's no wonder it's legs were so short.
So PS4's legs post-ps5 all depend on if it gets a price cut, and how far down they can go. A $49 console... I had no idea PS1 went that low. xD
Predictions (Made July 2019)
LTD: PS4 - 130m, Switch - 110m, XBO - 52m 2019 : PS4 - 15m, Switch - 18.8m, XBO - 4.8m 2020: Switch - 22m (Peak Year)