It's perfectly possible. The thing is, despite what people think, Nintendo can't support an HD console by themselves. They need third party support.
So, the switch will obviously experience draughts, just like every Nintendo system after the Snes. Even the sucessful Wii lost much of its support on its last years cause third parties jumped ship.
The underpowered home console gig just doesn't fly with developers and publishers. The Nintendo console for Nintendo games only works for fans. So, it's perfectly possible. Especially if you add the unsatisfied Wii U owners.
I dont think anybody has said that Nintendo can support a device all by themselves, but even with great 3rd party support a Nintendo device needs a solid output of Nintendo games because ultimately those are the biggest reasons to buy any Nintendo device.
Having a solid output of Nintendo games becomes increasingly difficult as development staff/time/cost increases and it's not feasible for Nintendo to provide adequate support to two seperate devices any longer.
Switch needs to be the successor to both 3DS & Wii U in order for Nintendo output to be adequate, if its just the Wii U successor and Nintendo releases a seperate successor to 3DS that is more powerful than Vita, Nintendo output will be poor on both devices and 3rd party support will be split.
The best move for Nintendo is to consolidate their software teams, their userbase & their 2nd/3rd party partners to create a single successful device rather than two seperate poor/mediocre selling devices.
Theoretically, s single device selling 50 million units of hardware & 350 million units of software is likely more profitable for Nintendo than selling two devices at 10-15 million & 35-40 million units of hardware with 100 million & 250 million units of software.