Beginning of 2019 is way too early. We already know that 2018 could be the peak year for the PS4 or close to it. Why would a company launch its new product when the previous one is still at the top? They will more likely wait at least one year for the PS4 sales to slow down a bit, it could be an announcement end of 2019, and a release in 2020 (or even everything in 2020).
The other question is the games. If the dev kits are being sent out in 2018, not enough games will be ready in 2019. And who would choose to release games on the PS5 when the PS4 is still selling truckloads? And a launch with only multi-gen games isn't ideal. You need everyone, developers included, to know that it's time for the next thing. 2019 surely isn't that time.
Developers will develop one more game for the PS4 or finish their current development, and then start working on the PS5. So one more cycle of games, which should take at least 2 years for the start of the next one. And they will lower the PS4 price even more in 2018 or 2019 to push units out before the next one arrives.
Holidays 2020 is my bet, with maybe a teaser at PSX 2019 and an announcement at E3 2020.
