I still don't think that PS4 will make only about 10M or even less after PS5 launch .. this is not PS3 .. it's not selling on the PS3 level, and is way more popular from PS3 to make just under 10M after the launch of PS5. If history is any indication and we take a look at PSP,PS2,PS1 and this that PS4 will be way cheaper than PS5 as PS3 was to PS4, PS4 should do at least around 20M after 2020, let's say 8M 2021, 5-6M 2022, 4M 2023, and 2M 2024. or some sort of it. So in total it should reach around 135-140M. PS4 can receive at least 2 more price points... and let's not forget what happened in 2009. PS2 receiver pricecut to 99$ and this helped it to sell another 20M from there on out. I am not saying that with 99 or 149 PS4 will do the same, but it will have effect. Price cuts often helps.
The only sony console to sell below 10M after the next gen launch was PS3. And this was 1 time case. Even the PSP sold more than the PS3 after it's successor launched. So I think PS4 has enough juice to reach 140M. But in no case will end up 120M lifetime.
PS3 sold 12'1M in 2012 (year 6) and 8'2M in 2013 (year 7 and PS4 launch year) according to VGC. PS4 has sold 14'4M in 2019 (year 6). If we take the same % drop for PS4 that will mean around 9'8M sold in 2020. Considering your point (and mine) that PS4 is more popular than what PS3 was back then and that the first half of 2020 is even bigger than what PS3 had in 2013 (and 2013 had GTA V.....), i can expect PS4 dropping less than what PS3 did in 2013. But don't get crazy, PS4 will definitively drop, and drop around 30% compared to 2019. If there is a price cut the drop will be softer but there will still be a drop. And as a we have already discussed in this site numerous times, Sony gets way more profits selling less at 300$ than a few million more at 200$, so don't expect that price cut either.
I only expect a price cut after or around PS5 launch. If PS4 is 200$ from 2021 it will help sales yes, but how much do you really expect an old console selling at 200$ at that point can do? After PS4 launch in Nov 2013, PS3 sold around 7M more. If PS4 is around 115M by the time PS5 launches, if sholud do better than PS3 but not by that much, that's why expecting around 10M more after PS5 is reasonable. PS1 and PS2 comparisons are invalid in the same situation because the markets and prices were vastly different as what they're now. There are no emerging markets appearing very fast in the picture nor absurdly 100-150$ pricecuts anymore.
So no, expecting 10M more or less in 2020, 5M in 2021, and then a few more years selling around 5M in total, is still the most realistic prediction. Don't be obsessed with raw selling numbers..., Sony doesn't. Sony cares only about profits and that's gonna be their main objective.