Its not likely to exceed 120mm if PS5 comes in 2020, as many suspect.
PS4 is on track to pass 100M by end of sept/oct 2019, and even if PS5 will be out worldwide by Nov 2020, PS4 will keep selling in 2020 and 2021 too. Sony is still holding a price cut, so it's highly probable if not sure that PS4 will pass 120M by the end of its life cycle. Now, if the price cut will happen in Sept 2019, and with all the AAA titles supposed to be released in 2019, you can place a bet with everybody that PS4 will be able to sell 7-8M from end of Sept to end of 2019, which means 107-108M PS4 sold by end of 2019. So, you are telling me that PS4 in 2020-2021 and maybe part of 2022 will sell less than (120M-107/108M)=12-13M ? you can't be serious. Even if my estimate is off by 30% regarding the Global by end of 2019, PS4 is still on target to easily pass 120M.
Yeah, I already granted that 120 million is realistic.
With that said, I do think that 120 million is pretty close to the correct lifetime total. Sales will begin declining noticably next year, almost certainly. A price cut will help, of course. As will any new big games. But, big games six years into a generation don't move consoles like they do early in the generation. Most of the buyers of those games already have a PS4.
We do have a bit of an unknown factor this generation that we haven't had in the past. That's the mid generation upgrades. Certainly many people have, and will continue to upgrade from regular PS4 to a Pro. I tend to think that that number will fall off pretty quickly though, as the people that would upgrade to a pro are also likely the ones that want to be in early in the next generation. So, presumably they would not buy a Pro if they expect a PS5 the next year. Nevertheless, the existence of the pro probably does help the lifetime sales numbers of the PS4.
This all of course assumes that PS5 is coming in 2020. If it doesn't come until 2021, the number will certainly be substantially higher.