I don't really buy devs are gonna let go of the PS4/XB1 userbase so quickly even if PS5/XB2 arrive in 2020. Case in point, last gen really took like 2 years to get going, and this gen will probably be worse because you have the PS4 Pro/XB1X. Devs are not going to want to abandon that userbase, and if Switch can comfortably port PS4/XB1 without too much fuss, it can stand to benefit from ports for quite some time, probably well into 2022/2023 when devs get really serious about PS5/XB2 only games.
By which time Nintendo can release another model Switch that can better handle those types of games if they want. A new high end model every 3 years works well for Nintendo IMO, that's enough of a gap, but it's also not too large of a gap where they're stuck with some kind of hardware that really can't run any of the modern games. There's gonna be a new major hardware model for all of Playstation/XBox/Switch every 3 years now give or take I think, it's just going to become the new normal in the industry (I'm not talking just New 3DS or PS4 Slim type revisions either, those will also be there, but full on Pro/X model refreshes).
A lot of those cross-gen ports will be just barely running on PS4/Xbone though, just like say MGS5, Middle Earth Shadow of Mordor, or Far Cry 4 just barely ran on PS3/360. And if they just barely run on PS4/Xbone, I wouldn't count on Switch Pro getting a look in.
Once again though, I think I'm gonna bow out of this discussion as I'm trying to be less negative on VGChartz.
Bet with Liquidlaser: I say PS5 and Xbox Series will sell more than 56 million combined by the end of 2023.