Weird, from the off, I read 2021 as the end of the PS4 production. In no way the direct correlation to next gens start time in any way.
For release, I expect Sony to keep their cards close to their chest anyway. With an E3 reveal for a Oct/nov release, or playstation experience for March. I'm happy whenever it launches, but I think late 2019 early 2020 makes sense. Too many projects have been moved from PS4 to PS5 for it to be later.