It depends on Next gen of course.
Consider PS2 it was at like ~112m at PS3 launch. 70% of its sales. The PS2 had great legs and late international launches like in Brazil.
Consider PS3 it was at like ~80m at PS3 launch. 90% of its sales. The PS3 had poor legs.
Now PS4 is at 91.6m. if we assume that is 75% of its sales then it will go on to sell ~120m. 80% would bring it lower, but I went higher due to PS5 not launching yet.
As I said I think 110m-120m is the safe bet. 105m-115m or 115m-125m would be pretty safe too, but a bit more risky. 100m-110m or 120m-130m would be even riskier, and so on and so forth.
That comparison with PS3...i don't agree. PS4 has ended its 6th holiday season. PS3 in its 6th holiday season was 2011 and PS3 was around 61'1M then. If PS3 ended selling around 87M this means it was around 70% of the total sales back then. So if even PS3 ended selling more than 25M in the following years, it's irrational to expect less for PS4. If PS4 sells 25M more like PS3 it will end around 117M at least. If we consider 71'6M as 70% of the total sales, that means PS4 will end around 130M.
PS4 won't end selling less than 120M in my opinion. There's nothing pointing in that direction.
Edit: The reason PS3 legs were so bad compared to PS2, were among other reasons the fact that PS3 lasted longer until its succesor launched.Last edited by colafitte - on 09 January 2019