Esp with what we know of how nintendo loves to abandon their consoles which has happened since the N64 days.
Even though Nintendo has a massive software output for one company it isn't enough to sustain a console all by themselves, even less two since they also have a portable to support. It has often been commented how this is a big problem for them.
3rd party support is needed to fill in and after SNES, it just wasn't there anymore on consoles (to some degree even with the Wii) so the N64 late years felt even more barren. Following their usual pattern Nintendo will likely move towards the U now and leave the 3DS to 3rd parties, who are still bringing many great games to the platform so the portable side doesn't face the same problem as you're suggesting the Wii U again will.
But Iwata has made several comments on exactly these issues. To summarize, the next Nintendo portable system will be like a brother to the console. To avoid the usual software draughts after launching a new system they will try to emulate the model on iOS and Android, where a new device can just use the same software previous hardware was able to run. And that they will do some new things that break previous patterns.
DS used modified (in many ways downgraded) tech from the N64, a console. The 3DS uses the same architecture used in the GC, Wii and Wii U, but again, in some ways downgraded and not fully compatible with either of them. What if the next Nintendo portable will share the exact same architecture with the Wii U? That way it has a great library right from the start and they can fully support the new system without abandoning the old one.
Consoles before the Wii U were lacking the second screen with its touch functionality that Nintendo handhelds had introduced with the DS. Adding it in the U allows for DS VC on Wii U. It also allows to make the same kind of games on portables and consoles again.
Iwata also said they want to strengthen DD and reward customers who invested in the Wii U. If they allow buyers of the next handheld to use all their digital Wii U purchases on the new portable, either by moving them to the new device (like DS to 3DS or Wii to Wii U) or even introducing a real shared account between Wii U and the let's call it DSU, allowing to play the same games on both devices with one purchase, imagine the positive hype by satisfied Wii U owners this would create for both the DD model and the new handheld.
3rd parties satisfied by their sales results on 3DS would have no reason not to support the 'DSU' and all games released for it on cart could also be released on disc for Wii U. Software problem solved on both devices. Also keeps the spirit of Miyamoto's design method to solve several issues at once.
Edit: This would also create the opportunity for Japanese devs to make games they can sell on the portable popular in Japan and on a console in the West, which doesn't share Japan's love for handhelds.
