It all depends on their ability to release titles for their biggest franchises in the first 2 years.
The Wii-U had an incredible head start over its competitors, but it didn't release anything worthwhile until the Xbox One and PS4 were already out. While I'm a Nintendo fan, there's no way in a million years you can compete with Halo, Destiny (lol) and GTA V with just remasters. They only really put effort into trying to sell trash fanservice: the game.
Yooka Laylee and Breath of the Wild will give the Switch a strong start, but if we don't see a Metroid, Star Fox (that's not yet another Lylat Wars reboot), Super Mario Galaxy, Donkey Kong, F-Zero or willing third party titles out and about before 2019, it won't be good enough on their behalf.
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Release titles are an important factor for sure but I think the most important one is price. If it will be at the same price of a standard PS4 or Xbone I can see it do well. But if it is $399 or higher it will probably not sell that much initialy, it must be cheaper than a PS4 Pro to succeed.
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Price is surprisingly not as much of a factor as one would expect. The Wii-U sold roughly the same amount of units as the Xbox One (and half as much as the PS4) off a similar price tag, despite being less powerful.
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AFAIK sales of the Wii U picked up only after a sharp pricedrop and the release of Mariokart and Smash, before that sales were lagging behind.