If I had to bet I'd say 2020. Sony is making so much money on PS4. They'll milk it (sic) as long as they can take it. The natural extension is that the PS4 Pro just becomes the default PS4.
PS5 is probably becoming their real 4K device, wont start selling it before PS4 sales decline. If PS4 sales slow down in 2019, they'll probably launch it in 2020."
(go to minute 47)
Frankly, that podcast is already 2 weeks old. The latest thread on NeoGAF I could find on this issue however is the one from July.
I'll stick with my 2019 prediction then...
Also PS4 Pro becoming default PS4 is nonsense from my point of view; Slim PS4 will be the cheapest to produce PS4 SKU in the upcoming years with immense ecomony of scale effects.