So in terms of "big" releases it would be something like this
2018: Detroit, God of War, Spiderman
2019: TLOU 2, Days Gone, Ghost of Tsushima
2020: Death Stranding, FF7R (if they ever release...)
What big games am i forgetting?
I'm pretty sure Kojima said that Death Stranding's target release was 2019. Remember that he got to use the Decima Engine from Guerilla Games. So although he was starting up a game studio from scratch, his starting point was a fully realized (and incredible I might add) game engine with extremely robust and easy-to-use developer tools. That saves a LOT of time, and essentially means his team could go right into "actual" game production.
Seems like a tactical decision. Releasing too close to RDR2 would be suicide.
I think this and maybe Spider-Man might be the reasons. All three are giant, 80+ hour open-world games that suck down a lot of your time. Releasing all three in the Fall might not be good, especially with 2 of them being your own first-party exclusives. I am thinking Sony will get Spider-Man out in Sep. if it is actually done by then, otherwise push it to a Dec. release to coincide somewhat with Sony's animated movie release then (and give RDR2 a little breathing room).
I am betting on Days Gone being Sony's early 2019 big release. Think Horizon's success from Feb. 2017. And then Last of Us Part 2 can release in fall 2019.