What i have clear in my mind is that, if PS4 sells 16M (is what i expect for 2019) and Switch ends selling 17-18M ( i expect 17'5M to be precise, but this is just an example) in 2019, that means PS4 is going to be ahead during most of the year again, and only when holiday season comes, Switch will catch up like what happened in USA last year with a tremendous December.
That's why i think 2019 is going to be close for PS4 and Switch. Switch will only be ahead since the beginning if it ends selling more than 3-4M compared to PS4. And i know, there's some people here that expect 20M+ in 2019, but if Switch ends selling just 20-25% more in 2018 compared to 2017 (like it seems is happening in markets like UK, France and USA), that means Switch is going to be around 16M at best. So, although another 20-25% growth this year could be possible worldwide, i really don't expect that happening.
So PS4 selling 16M vs Swtich selling 17-18M would mean another fantastic year of sales for both. Then, by 2020 PS5 will be announced and PS4 sales will drop a lot more until PS5 launches by November 2020, but PS4 should be near 120M total sales by then.
On the software side, Spiderman, is going to keep selling, but reaching 15M it's going to take time, so people should not expect that if Spiderman is around 10-11M right now should reach 20M easily lifetime. If it sells 15M only on PS4 it will be a monumental success.