OK, so then perhaps you'd say that Sega cultivated the audience better on Nintendo platforms after their own third party turn, except now that audience isn't even buying the most recent Nintendo platform. That audience exists somewhere, and I'm unconvinced that the entirety of it exists between Nintendo's own handheld and other mobile devices.
Retreating to handheld just seems to be a total non-starter. That market is troubled. Nintendo needs diversification of risk, they need multiple revenue streams, preferably streams that can grow independently of one another.
Sonic isn't really a good example because the newest game reviewed really bad and couldn't even sell anything on the 3DS. A majority of the people who played on the Wii and DS weren't traditionally gamers so many of them probably either stopped gaming, stayed on their previous devices or went to the next trend which is mobile gaming.