So the solution to having no games on a platform with a 50M install base is to make games for a brand new platform with zero install base?
Nah. There's no reason to think the 3DS will get a successor in 2015. The Vita isn't posing anywhere near as much a threat as the PSP was during the same time frame, and they just announced a smartphone -> 3DS connectivity thingy.
I'm guessing the 3DS will keep going as long as Nintendo possibly can, given that their next console and handheld are going to be designed to be "siblings"... ie I doubt the 3DS successor will launch several years ahead of the Wii U's successor.