More consumers will jump on board the PS4 train and buy more PS4s over the next 2 or 3 years before the PS5 arrives in 2019 or 2020.
Considering Microsoft is going to release the Xone Scorpio this year and the PS4 is still selling like hotcakes 2020 or even 2021 seems more likely for the PS5 to arrive. Why should Sony discontinue a console early that is very profitable and sells like hotcakes for the PS5 while the competition (Xbox since Nintendo is doing it's own thing) can't release their next console so early without pissing of many people. My estimate is that the PS5 releases during holiday 2020 after the PS4 is done releasing most of the major games in 2019 and it's sales are slowing down. During 2019 and 2020 the Sony devs can start developing for PS5 and give that console a kickstart into the new gen alongside the X2.
Please excuse my (probally) poor grammar