It won't beat 3DS. It might crack 40m. Even that might be lucky. Trying to convince people that a 720p tablet is worth $399 (rumored) will be the hardest sell Nintendo ever does. It won't have access to ios or android. If you look at the reveal video it doesn't show any functionality outside of video games. And according to some here, it won't even have a touchscreen and you will be required to use the controller nubs to navigate the OS.
So why would the average consumer buy a 720p tablet with no touchscreen at such a high price?
Some people on the internet seem to think that soccer mums, aunties, grandparents etc will be begging Nintendo to be able to buy the Switch like what happened with the Wii. But the Wii was extremely affordable as it was essentially an up-clocked Gamecube with no expensive large LCD/OLED portable 720P screen. Just because a reveal has X million views in Y amount of days does not correlate to day one or even lifetime sales.
It might be a mega success, it might be an utter failure. You can never really tell. The Dreamcast broke US launch records at the time and that console barely broke the 10 million unit barrier AND it had great third party support.