I principally agree with the Original Post but the answer depends on which year we take as the end of the console cycle. Most people forget that the current sales of the older consoles are usually not the total sales when the newer generation arrived, but they rather continued to sell for a few more years, which is especially true for Nintendo handhelds and Sony consoles. PS2, for instance, sold around 30-40 million after the release of ps3. So PS4 will continue to sell a few more years even after the PS5 comes out. It also depends on WHEN the PS3 will come out. Assuming it will be released at the end of 2020, we still have two more full years before release and probably 2-3 more years on extended support at the very least.
Sony themselves estimated that the PS4 will sell around 19 million this year, and assuming a further drop to 15 million in 2020, it will sell about 34 million more by the end of 2020, bringing the number to around 94 + 34 = 128 million, or let's just say 120-130. This is by the end of 2020, already surpassing PS3, and PS1. If it continues to sell about 10 million annually for 2-3 more years, it may reach the PS2 numbers.
Take a look at my ORIGINAL DETAILED SEMI- ACADEMIC ESTIMATES back in 2014 for PS4 and XB1
=> Summary, I had estimated, by 2020, PS4 would sell: 120-150 million, and XB1 :41-64 million
=> Yes, the ranges are a bit large but still CORRECT, and there were a lot of unknowns in 2014 (5 years ago!)
=> The 2014 estimate was predicting a minimum of 120 million sales by 2020 end, so yes, it will pass that point in 2 years. http://gamrconnect.vgchartz.com/thread.php?id=178341&page=1