I think even sony must be confused with the success of PS4 which might have thrown off their internal estimate of a new console by 2019, Sony must have expected Ps4 Pro to take off so that they can kill off the base PS4, but that didnt happen, So i think before Ps5 announcement, we will hear the announcement of Base Ps4 ceasing production and PS4 Pro taking its place which will be around 2019.
So Sony could delay PS5 to improve it further, which will also give developers some more time to work on end gen games port for the launch, also PSvr2 (if thats a thing) will greatly benefit from concurrent development, My bet is Fall 2020
I don’t think that Sony ever expected the Pro to take off. They always marketed it as a nice bonus for PSVR and 4K TV owners, but never as an essential upgrade like MS did with the X1X. They for sure know that price is the big deciding factor, not power.