Too early to tell, I want to know more about connectivity, media functionality and support first. First instinct is that it won't sell great though. For now, if it sells more than half of 3DS + Wii U (so 40-45 million lifetime), that should be considered a success in this market with a console edging into smart device territory.
The good news is that it can't possibly perform worse than the Wii U, but with the dedicated handheld having taken a pounding in the 8th gen, the total pool of potential customers could be smaller than many seem to expect.
I would categorize OP's 50 million lifetime as fairly realistic though, if slightly on the high side. 7 million first year also sounds like a decent guess, 7-8 million would be a good debut year, all things considered.
Edit; I notice that there's still confusion around whether or not this is a successor to both the 3DS and Wii U or only the Wii U, is that what a well communicated and marketed/presented console looks like? If we're confused like this, imagine consumers who are a lot more stupid than us on gaming. It also goes without saying that should this turn out to only the Wii U's successor, my estimates would be quite a bit lower, there's simply no reason to think that this (fairly ill conceived) contrapment will become another phenomenon.
