In a recent shareholder presentation, Satoru Iwata explained that they will still continue to develop and support the 3DS and Wii U after the NX launches.

Translation from the shareholder meeting:

When NX is launched, there already will be a certain volume of Nintendo 3DS and Wii U hardware widely existing in the market, so from a software business perspective, it would be highly inefficient to stop releasing titles for Nintendo 3DS or Wii U right after the launch of NX […] As for Wii U, we will continue to make efforts, as it is a priority within the company to think about how we can satisfy the consumers who purchased this system to the maximum extent possible.

You can find the full shareholder presentation Q&A here.

The NX is reportedly set to begin manufacturing this October, with a potential launch next year.