Nintendo can probably afford the occasional stagnating console; what it cannot afford (in the long run) is the slow dilution of its core franchises.
The next shift.
As a follow-on to the 3DS generation, they might want to consider developing a snappy clam-shell smart phone that is primarily a portable gaming console. Think about a low-power e-ink screen (or similar interface) for phone calls and other minor tasks in the closed position. Maintain and improve the Streetpass feature set. Do not use Android. Do consider Ubuntu or Firefox OS.*
Why this could work. Adults are unlikely to give up their existing smart device/app collection but are probably already playing their portable gaming device(s) in the comfort of home anyway. Look into it. Children in the market for their first smart phone would be the primary target for a converged experience. Their minimum third-party app requirements will be lessor than their parents and those few they’re inclined to use could, in most cases, be ported.
EDIT: A less expensive phone incapable version might be released alongside the flagship device. But both versions should be able to communicate via some form of root-level walkie-talkie app.
Contrary to some contemporary opinion on the subject Nintendo does indeed need to remain in the proprietary hardware space.
Final note. The legacy of the Wii U (including the supposedly ‘controversial’ tablet controller) has not yet been written or (I’m inclined to think) accurately foreseen.
*Consider Tizen with care. It could be a candidate if a hardware licensing deal is needed. But be cautious about this approach as Samsung will one day [I predict] try to acquire the company wholesale. Nintendo, even as an independently operated subsidiary, would lose something.