Splitting the user base?
The WiiU is already dead, 13 millions console sold in 3 and a half years.
Maybe it will split that in half, maybe there will be 6 and half millions WiiU owners that won't buy a NX, and that's a big if.
But Nintendo's objective are far beyond those number, you can be sure they aim at 50-80 million NX sold
So yes they are killing the WiiU and taking a shot with NX, it's totally worth it