Logically it should be. 2019 should have some amazing games, but PS4 should end 2018 selling just under 20m consoles this year, which means total hardware sells will be over 90m units sold. At that point that console is selling to the $199 and less gamer (which is generally a small market, since $250 is the magic entry price now), and people buying their 2nd and 3rd PS4 (which again is generally a small market).
The one thing that might help them stay ahead of Switch in 2019 would be current PS4 owners upgrading to a PS4 Pro, in anticipation of a PS5 in 2020 / 2021, just to have something to hold them over til then.