Well, they are milking their franchises. Last year we got two different NSMB games. Year before that we got Super Mario Land and Super Mario Kart, as well as Zelda Ocarina of Time and Zelda Skyward Sword. Year before that we got Super Mario Galaxy 2. Year before that we got New Super Mario Bros Wii. Etc
Nintendo does rely pretty heavily on franchises, and the general point is not that we only get at least one particular flavor of the Mario franchise ever year, but that Mario is featured in at least one game every year without break, often times within months of each other.
Of course, what people seem to forget is, Nintendo is not the only company that relies on names and franchises to sell software. Pretty much EVERY publisher does what Nintendo does, with the same if not more frequency. Yearly Assassin's Creed, yearly Call of Duty, yearly Halo, bi-yearly Gears of War. Sony, too, released two different Uncharted games within months of each other. Handheld version was made by a B team much like NSMB2, but they were still essentially the same kind of game, just on a different platform. People didn't complain, either. Little Big Planet got two games this year (including a cart racer) and one last year, one two years before that and another one the year before that. Don't be surprised if we get another Halo, another Dance Central, another Uncharted, and another LittleBigPlanet game announced this year.
The question isn't whether Nintendo milks their franchises. They do. A given series of game may only get one or two releases on a platform in it's life span, but they still milk franchises. The question is why people are singling Nintendo out for this complaint, when the industry as a whole, including companies, journalists, and consumers, get excited for, mostly make, and mostly buy nothing but sequels. This isn't a Nintendo problem. It's a game industry problem. And it's one that isn't really a problem because these games are still often pretty damn good, which is why people buy them, which is why companies make them.