Another point would be to consider the amount of current gen blockbusters that we are still waiting for.
GTA6?  Elder Scrolls 6 anyone?
You'd also kill sales for a successful console and get a backlash from those that feel current gen still has a lot to give.

2020 launch seems likely, but it makes no sense to rush it.

2019 seems too soon.
Seeding dev kits for non existent parts?
Navi, when its the last of GCN and has no room for a mid-cycle revision?

AMD locked in, when there are so many potential options for what a playstation could be?

Seems way too soon.  I'll be paying attention to rumours next year, when the next gen is clearer, but for now I doubt anyone would lock themselves into buying parts that no one knows much about.

Release in 2020 would be most likely as it sounds "big" and "bombastic" and would indubitably fuel the hype train far better than 2019. Probably will be announced late 2018 or early 2019.

I'm hoping the focus shifts to locking games at 60 frames per second as opposed to more 4k/ 30fps slideshows. Speaking as a life-long console gamer myself, I feel framerate-centric marketing is long overdue in the mainstream consumer space. 120fps+ would be ideal, but I am trying to be reasonable here.