2019 is if they want to punish Xbox...
People that just bought a Xbox One X (which basically launched 2018 (late 2017)) would feel burned, if Playstation 5 / Xbox Two launch in 2019.
2020 is if they want to milk in the success & profits:
This could lead to 130-140m console sales down the line, and tons of money made off of this user base.
I think 2020 is more likely tbh.... but I wouldnt rule out a 2019 launch.