If 2020 becomes the peak year, I see Switch outselling Wii and probably PS1 as well. If people in this thread are right and somehow Switch doesn't peak until next year, it's outselling the Gameboy and PS4 as well.
What's stopping it from peaking this year and outselling ps4 and gameboy?
You mean what's stopping it from peaking next year? Not much, I guess, if the rumors about the upgraded hardware are true. Nintendo just needs to announce some good titles for the holiday season. BotW 2 would probably be enough. Announce that at E3, and you can just imagine the hype when out of nowhere BotW 2 is less than half a year away. I still say it'll be pretty shocking to see it peak in 2021, if only because no Nintendo console has ever peaked in its 5th year, at least for the ones we have data for. Not even the DS did, it barely peaked in its fourth year. Maybe the Game Boy did, but the Game Boy Color felt like an entirely new system, and made the Game Boy last through two console generations, so it was a phenomenon unto itself. Switch repeating that would be a sea change for the gaming market, drawing a clear line in the sand of the point where Nintendo went from being the secondary console you own in addition to your preferred console of Xbox and Playstation, to being the preferred console with PS and Xbox being the secondary choice. Not just a portable supplement to home console gaming like Game Boy and DS were, but the primary experience, the only apt comparison being the first two Playstations. Even if you feel you can predict it, it still feels pretty astounding.
Let's say Switch ends up 20% this year, and another 20% in a hypothetical 2021 peak year. That would be 24.25 million this year, and 29 million for the peak. That's DS numbers, it would be less than a million shy of DS' peak at that point. If it peaked in its 4th year, that would match PS4, but since Nintendo consoles tend to decline faster than the other big platforms, it would probably come up short. If it peaks in 2021, it could do a typical Nintendo decline and still end up past PS4's likely lifetime sales by the end of its sixth year. It would have a non-zero chance of passing the DS or PS2, but I'd probably estimate about 135 million.
Now if you're asking what's stopping it from outselling the PS4 and Game Boy even if it does peak this year, well nothing, aside from past data for past Nintendo consoles. Which to me means nothing's stopping it per se, but I still wouldn't bet on it.