Sony can sell the current PS4 slim at 199$ without loss imo, and probably tiny bit of profit.
Plus with this many units sold,.... does anyone expect 3rd party to just abandon it, the day PS5 launches?
3rd parties didn't really abandon the PS3 either. Destiny, Watch Dogs, Shadow of Mordor, AC Rogue, FF XIII Lightning Returns, South Park Stick of Truth, Dark Souls 2, Metal Gear Solid 5, Alien Isolation, Borderlands Presequel, CoD Advanced Warfare/Blackops 3, Far Cry 4, Resident Evil Revelations 2. Though you're right that PS4 support should be better.
$149 might be possible, though I don't expect we'll see it happen. If it did drop to $149 though I could see it actually beating the PS2. We're not even down to $249 yet though.
Predictions (Made July 2019)
LTD: PS4 - 130m, Switch - 110m, XBO - 52m 2019 : PS4 - 15m, Switch - 18.8m, XBO - 4.8m 2020: Switch - 22m (Peak Year)