I wouldn't predict that this will do worse than Wii U, because it is also successor to 3DS...
But I would say there is strong chance of it not matching 3DS sales, never mind 3DS+WiiU.
At that point, why bother? They are better off going 3rd party NOW instead of delaying it.
Because the longer they delay and their consoles decline,
the smaller the market that even remembers and connects with their successful past.
Nintendo has focused on kids being their market, yet kids will be least engaged with their brand.
Parents aren't going to buy this as kids' console when it costs same and is weaker brand with less games.