They did try to change the 911 several times. The 928 was supposed to replace it, 911 buyers protested, Porsche listened. They came out with the 996, 911 buyers said it didn't look enough like a real 911, Porsche listened. Many Porsche purists think the best 911 is the 993, the last of the air-cooled beasties.
We like the heritage, the instant recognition, the quirks. It's not always about having the latest greatest most modern styling, there are more important things, like driving.