A big reason for PS1 & PS2 having such long legs is that they released much later in many developing countries. This isnt nearly as true for PS4 so its legs likely wont be nearly as long.
It will be better than PS3 but much less than PS1, I'm going to say 15-20 million.
Thrown in easy piracy and reliability issue for PS1 and PS2. I bought units even after I had their successor lol.
PS3 and PS4 have been good too me. The only negative I got to say about the PS4. sometimes it spits discs out at me when I haven't turned it on for months. Guessing after some update has happened in the background perhaps.