If they wanted to do the same thing they did with the PS2, they probably could do it, but they simply don't want to.
The PS1 sold another 30 million after the PS2 launched, the PS2 obviously sold a ton after the PS3 launched, but manufacturers don't care about the low end market that much any more.
PS1 and PS2 sold THAT well in the latter years due to late launches in many countries including major ones. PS3 only sold 6-8m after the PS4 launched, and PS4 is heading towards the same trajectory.
Ps1 sold well after PS2 release because it was $99 (launch price for PS2 was $299).
PS2 sold well after PS3 because it was released in many developing countries AND was $129 (against a $499/599 launch price for PS4).
When PS4 launched at $399 on 2013, the PS3 was $269/299.
If the price of PS5 is around $499 and Sony want to drop the price of PS4 to $199 (slim)/$299(pro) it could make more than the 8 million units the PS3 sold after PS4 launch.
As for 2020, PS4 is tracking 2 million units more compared to what PS3 was doing in the same time gap back in 2013.
Im not saying that PS4 will sell Ton of millions more, but saying that PS4 is following the same trend of PS3, at the moment at least, is not true.
But all depends if Sony wants to slash the price of PS4 to $199/299 or not.