Franchises will always get milked. Sometimes it works, sometimes they need to let it die with some dignity. Example;
- Final Fantasy was the ultimate non-Nintendo franchise at one point. It got all the way to ten numbered games before it started to suck. Now it's a joke.
- Sonic the Hedgheog was untouchable in the early days, then he fell from grace, but recently he's not doing all that bad. Generations was pretty decent IMHO.
- The Pokemon franchise seems utterly immune to the regular rules of franchise milking, as it stands with Tetris as a series that is released more or less unchanged pretty much every year and yet people still praise it.
Some franchises can keep things fresh, like Mario. Others get stale after a while. It all depends on how well the series adapts to keep the player from playing the same thing over and over again. It also depends on the fanbase. Some games like Pokemon get away with murder because they have a wide audience, so they'll still sell even if they are getting samey.