Over a 20+ year timeframe Nintendo has released the following main series Mario and Zelda games
Super Mario Bros. (NES)
Super Mario Bros. 2 (NES)
Super Mario Bros. 3 (NES)
Super Mario World (SNES)
Mario 64 (N64)
Super Mario Sunshine (Gamecube)
Super Mario Galaxy (Wii)
The Legend of Zelda (NES)
The Legend of Zelda 2 (NES)
The Legend of Zelda: LttP (SNES)
The Legend of Zelda: OoT (N64)
The Legend of Zelda: MM (N64)
The Legend of Zelda: WW (Gamecube)
The Legend of Zelda: TP (Gamecube/Wii)
7 games for each series which works out to (approximately) a game every 3 years ... This is not exactly "Milking" a franchise.
Certainly, Mario (as a character) appears in a lot of games ... but Nintendo is using Mario more as a celebrity actor to draw attention to these games. I suspect (but could be wrong) that this will become more common in the future and large publishers will use their mascot characters in as many different types of games possible to increase sales. We have already seen this happening with Sega's abuse of Sonic, and Microsoft is getting into this with Halo Wars.
