Ps4 is on the 70 millon now. I guess we can expect 90 at least for next year and more than 108 by end 2019. That tops PS1 at less than 105 millon and Wii at arround 102.
Reasons: XBox one X is faster, but too expensive and this holiday ps4 is still doubling xbox in numbers. Xbox doesn't have great exclusives in the near future.
Switch launched Zelda, Mario and Karts. Besides that Ps4 managed to pass 20 millon this year. I don't see Switch taking many customers from PS4. The way I see it Switch is not a home console that can be portable, Its a portable that can be conected to a TV. So my point is that switch has the people from the wii u + some casuals + all the nintendo 3DS mobile gamers that are upgrading to the best mobile console ever with great games. Customers are mobile gamers and thats why its exploding in japan.
The ps4 has the pro, it doesn't add much on sales, but one customer can buy it and it will be counted as 2 PS4 sold. (I have 2 ps4 pro and sold 1 standar, so i bought 3 through time). That did not happen with other consoles before.
Comparing to PS2, PS2 sold close to 160 millon consoles, and it will be difficult to reach that amount.
But PS2 had an advantage. When PS3 launched it was really expensive at 600 USS. If PS5 goes out on 2020 or better 2021 and at 600, then I can see the PS4 going for the 1st place in history.
Also PS4 can lower its price. At 150 US could sell very well in developing countries and as a casual console too.
There is a posibility that also PS4 pro will run PS5 games at lower resolutions becoming a cheaper option for many.
Also 4k TVs will reach more market in 2020 and a cheap pro will be a nice cheap upgrade for people selling the old ps4. This also is the 1st time that happens. Imagine you can sell your ps4 at 125 - 150 uss and buy a pro at 250 if you care about visuals and will count as another PS4.
Game wise, 2018 will be big. I can see Dead Stranding and Last of Us 2 as 2019 big sellers. I can expect another From Software game, another GTA and another battlefield placed on WW2 or modern days after COD WW2 hit.
Another little boosters will be cheap PSVR and the fact that the network of friends and where they play is more important than ever on the decision of buying a console.
Coming back to the post I can easily think that PS4 will become 2nd best selling home console in history by xmas 2019.