The PS2 has several advantages that the PS4 does not in terms of extending sales into the succeeding generation.

1). Price. PS2 was $129 in the US when the $499/$599 PS3 launched.
2). Market dynamics. PS2 enjoyed an astounding market share with very little global competition from other devices.
3). New markets. PS2 had the benefit of gaining traction in markets that had not yet really gained mainstream video game access.
4). Counting. PS2 production was how sales were tabulated by Sony back then. Not units sold to retailers. So any warranty replacements, retail demo units, developer kits, etc...were counted as part of that 157 million. Sony changed this practice to the industry standard in 2007 (I forgot which quarter).

Exactly the sort of post I was looking for the moment I saw the topic.

That sure sounds like a can't, then.

This.

Except you are a lot more optimistic than I. Based on the current situation, 125m is the maximum. An expensive PS5 would change my thoughts on that matter a bit.

I feel the ps4 sales will stay stronger than the ps3 did simply due to the price of the ps5, there's no chance of the ps4 selling another 40 million form here

Not happening. The price gap between the PS2 and PS3 was massive, and PS3 stumbled out of the gate.
There's still a lot of unknowns about the PS5, but most of what we know is promising. The next-gen transition should be a smooth one.

Some years ago, this was dabatable.
Right now is almost impossible for PS4 to outsell PS2, even with PS5 being 500$.

I'll go for around 130 million lifetime.

It seems that many of you forgot that PS2 sold multi-million units late on its life cycle in developing countries because of PIRACY. In the Philippines, you could buy a brand new PS2 console for $125-$150 and a pirated DVD copy of PS2 games at only 50 pesos ($1). Cheap price of games made PS2 a blockbuster console in Southeast Asia. But nowadays, you can not find pirated copies of PS4 games. A minimum wage worker here only earns 12,000 pesos (about $240) a month. So ordinary Filipinos who previously could afford to buy PS2 find the PS4 luxurious. Thus, only the people who belong to higher socio-economic class can afford to buy a PS4. With that, PS4 can never reach PS2's numbers.

As the article pointed out, the only chance the PS4 has of outselling or even coming close to the PS2 is with a price drop from $299 to $199 this holiday season and more price drops and deals after that. Basically everything needs to go right for there to be any chance, while the PS5 launch is as bad as the PS4. 

At one point I think PS2 was sold for $80 in the 7th gen new. It might have been a sale but still, that's super cheap. I don't see PS4 going for $99 or esp $80 anytime soon.