Don't think Switch will do PS4 numbers, but I do expect Switch to surpass Wii and PS1. PS4 might do 130 million so Switch could very well end up like 20 million behind it when all is said and done. The great thing Nintendo has got going for it is that it's closing in on 3 years and hasn't done a price cut yet. Next holidays maybe they'll ready two huge games (2d Mario + BotW2 ??) plus first price cut plus maybe a Switch Pro all ahead of the Micro/Sony launches to keep momentum going for the Switch through the new system's launches.
They've also got a bunch of handheld franchises that have yet to be brought to the Switch that could help fill up the next 2 or 3 years. Plus Animal Crossing coming soon, and waiting on stuff like Metroid, Pikmin, Fzero????, Star Fox??, Paper Mario, maybe a new DK, and several other sequels to games that are already on Switch (I'm thinking at least 3D Mario, Mario Party, and another Pokemon).
This is off topic now, but I would hope the success of the Switch gives Nintendo the money to get 3rd party AAA support next gen. If Nintendo now sticks to the mid-gen release schedule started with the Switch, that means they could release Switch 2 say like 3 years after ps5/scarlett. Perhaps that time difference plus their profits from Switch would allow them to make a Switch 2 that is close enough to the other two systems in power as to be able to get downgraded but still full versions of 3rd party AAA games releases. You know like drop framerate, rez, some graphical bells and whistles but keep the game very playable and the same overall. If Switch 2 could do that it would be the ultimate version of what Nintendo is doing - hybrid console, full Nintendo focus on a single system, with AAA 3rd party support as well to also compete with those tens of millions of gamers who won't even consider the current Switch.