It'll be three years from now, four years at the most before we see a 3DS successor announced, and five years before a Wii U successor. If they were to announce a device that replaced a home console and a handheld system it would either A) be too early because Wii U wouldn't have been on the market long enough or B) too late because 3DS will have been on the market too long and sales would have long since dried-up.