If late 2019 is still on the cards, 1st/2nd party is already working on a 1 or 2 launch window games [Horizon 2 by GG is a prime candidate], but true point of no return is deployment of knowledge and devkits to 3rd parties. That can happen before GDC in March [like it was with PS4] or before E3 2019 [since dev tools are now very streamlined and there are no big architecture changes].
And if PS5 launches in late 2019, IMO PS4 Pro looses its reason to be on the market. At that point, Pro should get discontinued and fully BC-capable PS5 slotted in its $400€ place. After all, it will boot PS4 games in Pro mode with high-res rendering & all. Base PS4 can continue selling [hopefully with 7nm APU and pricedrop] until all currently announced 1st party titles are out [some will be 2020 most likely] and major 3rd party support switches to gen9.
I think similar should happen if PS5 is targeting Q1 2020 launch [trying to emulate Switch launch, with strong spring/summer sales before arrival of big holiday titles].