I don't think market saturation will be a limiting factor here as once you drop to $250 and especially $200 you reach whole new segments of the market for whom $300 was too much. I think people are really underestimating how much a pricecut will do for PS4.
And while I don't expect PS5 to repeat PS3's mistakes, I think it's likely to be $500 which would leave room for PS4 to coexist for the first few years of the PS5 as an affordable $200 entry level system.
I find it hard to see it doing less than 130 million lifetime when its nearing 100 million while still selling decently at $300.
The PS4 has been officially available for less than $300 on multiple occasions, including Black Friday deals that encompassed ~1m consoles per year in the USA alone. A significant chunk of the people who have been waiting for a cheaper PS4 has already been addressed, so a permanent price drop to $250 wouldn't have much of an impact anymore. An immediate drop from $300 to $200 is really farfetched because Sony insisted on $300 for so long.
Regardless of what the PS5 will cost, the market will be flooded with used PS4 consoles to put money towards PS5 purchases. That will limit the number of new PS4 consoles that can be sold after the PS5 has launched.
130m is definitely at the higher end of the range for PS4 lifetime sales, not the floor. The number of high profile releases has slowed down this year and 2020 isn't going to be better, plus 2020 will see the marketing focus shift to the PS5 during the holiday season.
Range for PS4 lifetime sales is 120-135m, because the current fiscal year can create quite a bit of swing. If it's 16m like projected, the range can be narrowed down to 125-135m next year. If it's worse, the ceiling will drop depending on how much worse. If it's better... I doubt that will be the case because in the current state a permanent price cut is already a requirement to get to 16m.