I can understand how people get the impression that Mario is a milked franchise, but I also think that Nintendo has the right idea with how they're using it ... I will try to explain from a few perspectives.
When you look at videogames from a developer's perspective you will realize that a major problem is maintaining consistent quality, while releasing the product on time, and not breaking the budget. Even Wii games (if poorly managed) run the risk of being 12 to 18 months behind schedule, and running (up to) $20 Million over budget. The most expensive part of development (by a wide margin) is the cost associated with producing and animating the 3D assets that are used in game; on the Gamecube, PS2 and XBox you had development teams of 40 to 80 people, and the artists made up 75% to 85% of these teams. By producing a 'Mario' game Nintendo has the ability to reuse a massive number of graphical assets, which should (dramatically) lower the number of artists they need to produce a title, keep the number of new assets needed to be created to a minimum, and should enable the artists to focus more on producing high quality reuseable assets. Basically, if you have a really good turtle shell model why would you need to model another one when you create a new mario game?
From a Publisher/Retailer perspective the problem is always convincing retailers to stock games that will sell, and for a retailer to avoid devoting shelf space to games that will not sell. Mario sells ... You put that happy plumber's face on Brussle's Sprouts and they will sell ...
From a (typical) consumer's perspective you're faced with a wall of games to buy at your local store with little knowledge of what any of the games are or what their quailty is. Earlier today I was thinking of buying a racing game for the 360 and went to buy Forza 2 which they didn't have; there must have been 12 different racing games available, and I recognized several based on their title but knew very little about most of them. After decades of Mario games gamers know what to expect from these games, and know that they are typically very high quality and a lot of fun; certainly, you can debate the quality but every Mario game is enjoyable (for what it is) for 20+ hours.
