This wouldn't make any sense at all. They've already merged their software divisions, no point for two systems on the market to hurt each-other.
Nintendo has to screw up horribly for Switch to fail, even if it doesn't sell well they won't dump it and release a new successor to the 3DS, they'll slash the price and remove the dock. Hold me on this - there won't be a 3DS successor during the Switch's lifetime. It honestly baffles me how people are even considering this a possibility.
They're only supporting the 3DS because they still had games in development for it and they just finishing up; besides they don't mind milking it till the end, this is why they're marketing the Switch as a home console replacement. The Switch's a hybrid, and it's replacing both systems, Nintendo realised this makes a lot of sense for their software lineup and audience.