What's fascinating is how ya'll seem to think we're saying "soon" as in tomorrow. 2020 (the latest we see the PS5) isn't soon, particularly when we know Sony's 2018 line-up and can see part of their 2019 stuff too. Several of their first-party devs are going to need to get on developing for the next system soon--like this year--if they'll be ready for launch in 2020, nevermind 2019.
I agree with the idea that it's likely to be a 2020 console. With that said, I don't get the conclusion some reach that they're coming to an end with PS4 game development. Let's just look at their current lineup.
Matterfall - August
Uncharted: The Lost Legacy - August
Everybody's Golf - August
Knack 2 - September
Gran Turismo Sport - Fall
LocoRoco 2 Remastered - 2017
Horizon: The Frozen Wildlands - 2017
No Heroes Allowed! VR - 2017
PlayLink games (Hidden Agenda, That's You, Knowledge is Power, Singstar, and Frantics) - 2017
God of War - Early 2018
Spider-Man 2018
The Inpatient - ?
Bravo Team - ?
Days Gone - ?
Death Stranding - ?
Detroit - ?
Shadow of the Colossus - ?
Wild - ?
Dreams - ?
Of those with no release dates, i'd expect The Inpatient, Dreams, Detroit, and Bravo Team to launch next year. Perhaps SotC as well. Which would just leave Death Stranding, Wild and Days Gone for the rest of the PS4's life. And even if Sony launches in 2020, their history is one that suggests they'd still support the PS4. With the PS3 they still launched TLoU and GT6 the same year as the PS4. I just can't see how they aren't sitting on a decent number of unannounced PS4 titles. Yoshida basically alluded to that when he said we'll hear more from their studios later this year.