They are probably close to 20 million sold through in a year and a half and without any price cuts, big bundle or big deals. There is still not much of their biggest sellers out. Switch still has no exclusive Monster Hunter/2D Mario/Real pokemon game/good party game/exclusive mario kart/Animal Crossing/Smash.. they have so many marketing possibilities with the thing.
Sales will exodes during the end of this year and 2019 has already more big games than this year. Once they are willing to drop the price and make bundles/collectors/deals it will be unstoppable. It will probably get a revision too.
Honestly it might not outpaced the wii in its prime (but i think it will next year tbh) but the Switch has something the wii didn't have, a good reputation amongst gamers. Families love the Switch and once they give them Mario Party and 2D Mario they will love it even more. Gamers love the Switch and once they get Smash/Core pokemon games/Metroid etc, they will still love it. Everyone loves it and it wont get shit on later on like the wii did since its able to get some 3rd party games and propose a good gaming experience.
So yea, will have big peak years in the near future and will last atleast couple years longer than the Wii, so i don't see it not outselling it.