Horizon 2-After doing Killzone as a launch title, I don't think Sony wants to risk this large of a game again. An early 2021 release makes more sense so the game can be delayed an extra month or two if needed.They're definitely not going to want to rush the game at the expense of the quality.
But if they are confident in its quality, releasing it day 1 would be a boon.
It's pretty safe to say that Killzone Shadow Fall was the best selling Killzone game by a solid margin.