The PS4 can't keep selling this well forever. It's clearly past its peak in Japan, and in the U.S. it's only up YoY this year because of GoW, Spider-Man, and RDR2 (ignore April, September, and October, and its baseline is down slightly). Sooner or later, Sony is going to run out of people to sell it to, and eventually no amount of price cuts will keep a system from irreversibly declining (we also never see deep price cuts this late into a generation, so $50 is the most I'd expect to see if we even see a price drop). Even Sony is predicting post-peak soon, if their shipment projections are any indications. Their mid-year sell-through update also suggested a YoY decline globally, which given sales in the U.S. & Japan there had to be noticeable drop elsewhere. The first few months of next year ought to give us a good indication of where the PS4 is headed, but I think it's about to enter its twilight years. I think 2019 ought to be down noticeably from this year.
Meanwhile, the Switch is still young and has nowhere to go but up. Nintendo has yet to issue a price cut, which could happen next year. Also, Gen VIII Pokemon is due out next year, and that's guaranteed to be a major system-seller. The Switch ought to pull in serious numbers next year.
The possibility of the Switch coming in #1 in 2019 is not too far-fetched.