PS4 might sell 19-20 million this year, maybe more. But it's probable that it starts to slow down in 2019 beyond. The Switch on the other hand has only started out, and has many years left. There are those who already think Switch will outsell PS4 this year but assuming it doesn't, I myself think this is the last 18+ million selling year for PS4.

Do you think Switch will take the reigns as sales king in 2019 and going forward?

Nope

Unless Nintendo has issues and is forced to delay certain titles, 2019's Switch lineup includes Pokémon (mainline title, no GO features), Bayonetta 3, Metroid Prime 4 and Star Fox Grand Prix, add to that Animal Crossing or a 2D Mario and you have a 20M+ year

I think last year was the last year PS4 will outsell Switch, but it will be very competitive.

Nah, I think PS4 will edge out Switch in 2019.

Probably not, but the possibility exists.

Sony has forecasted 16M PS4 for the financial year 2018. This includes Q1 2019 but not Q1 2018. Even if Sony manages to outdo it's projection, 19-20M seems unlikely for the year 2018. For Switch on the other hand Nintendo expects to sell 20M. Again financial year, not calendar year. It might be close, but maybe the Switch does outsell the PS4 already this year.

Switch needs to kick it into high gear if it expects to beat PS4 this year. Already a 1m unit head start for PS4.

Ya the odds of PS4 exceeding Sony's forcast is pretty high but it would be realy weird for them to undershoot it by that much. With this being PS4's 5th full fiscal year they should be pretty on the mark with their forcasts.

Yes, I expect 2019 could be Switch's peak year, so PS4 is definitely going to fall behind that.

Switch sales in 2019 should be 20-21m imo.

Whereas for PS4 sales I'm expecting

2018 -   ~18.5m
2019 -   16.5-17m