The Wii U is great for emulating DS games. But no future console will be able to emulate DS or Wii U games without one touchscreen plus one other screen.
The Switch has a touchscreen, but you can't use it when it's docked. No game will be able to support touch controls while the Switch is docked – unless you have another controller with a touchscreen. Which I already have, made by Nintendo.
So if they support the Wii U Gamepad as a controller, and maybe release a lite Gamepad for $99 specifically for the Switch, we'll get a lot more gameplay possibilities. We can emulate the Wii U catalog as well as the DS catalog.
And we'll be able to do 2-player local with 1 Switch and 1 Gamepad instead of requiring 2 Switches.