At this point late in its lifetime it's not wrong to assume that Nintendo's working on a new handheld, plus, with the Wii U going out next year (make space for the NX) it's all coming together. 3DS will definitely last a while longer than Wii U because it's a successful handheld and Ninty wouldn't want to upset them.
Exactly, and it makes even more sense if you think of it in terms of servers. After all, with the NX coming out Nintendo won't want to maintain two sets of servers, one for the Wii U and another for the NX. However the problem with shutting down the Wii U servers is that the 3DS also uses them. Thus, a new handheld to replace the 3DS would be required before they can shut down the Wii U/3DS servers.
