Sony has been pretty clockwork when it comes to a release schedule.
(PS1) December 3, 1994
(PS2) March 4, 2000
(PS3) November 11, 2006
(PS4) November 15, 2013
(PS PRO) November 10, 2016
So even with the PS Pro you could view that as a halfway point of the console's life cycle. It would be a safe bet that in 2019 you will see a new system announced at least. I would doubt we see an actual release next year but who knows.