The 3DS is the right age to get a successor. The Wii U arguably should last at least one more year, if we go by industry standards.
Marrying the NX handheld to the NX console is a wonderful idea. The handheld market is still healthy worldwide, but specially in Japan. I think the New 3DS decent sales proved there is still demand for a dedicated gaming device on the go.
So its safe to assume the common consumer will get the NX handheld (Lets call it NXDS because reasons), it should be at worst a moderate success. So from there, Nintendo has to convince the consumers that are satisfied with the handheld that is worth their time to consider a NX console to go with it.
If its shown to a consumer that all the games they already got for their handheld can be easily upgraded and improved with a home device, it will be a huge incentive for them.
Here is an example:
A consumer gets NXDS and Super Smash Bros 4 NX at launch. Loves the game, plays by himself while on the go and with other people when they also have a NXDS. But one day he has friends over and Mario Party 2 on the Wii vC is not an option because his girlfriend broke off with her last time they played that.
So he remembers that he can get a NX Console and all he needs to do is insert the Super Smash Bros 4 NX game he already has, dusts off the Wiimotes and now he has a Smash Party for up to 8 players, instead of just 1. He doesn't need to buy a game he already has, that is, in his mind, $40 bucks he just saved on his new purchase.
Another Example.
Pokemon Z releases and its awesome to play on the NXDS. The game looks great, but sometimes the consumer feels like sitting down and enjoying Pokemon on his big screen. Plugs in the NX Console, in goes the game. Now you have a main Pokemon game in HD.