You are wrong. There are a ton of companys that sell the same product year over year that is identical in look to its predicessor.
Take for example the IBM/Lenovo thinkpad. That laptop has looked the same since the 90's and they only change the internals. That continues to sell very well amongst the business community.
or
Porche has had the 911 running with the same body style for like 60 years now.
no, not identical - similar but not identical.
looking at the IBM ThinkPad, you have to look at the market in which IBM is catering/selling too. This is not the typical consumer electronic, but a business machine. I could type up an entire essay about the ThinkPad and why I think IBM has not changed the style, but that would be way to long. Ill just say it hasn't changed because the ThinkPad as become a brand on its own. It has become the brand of choice for the corporate setting.
Now looking at the Porche 911, same thing, it is a product that has been around for quite a long time. It operates in a niche that signifies wealth, status, etc. The classic look that Porche would be dumb to change.
Finally we get to the Apple Iphone. This product operates in a market where aesthetics have become highly important. People want a nice looking, sleek phone. Turnover is extremely high in this market, with new designs released every few months (not by each company). Now the examples you have used are only usable because they have not been changed for quite a long time. So, if you want to use your Porche and ThinkPad example, you will have to agree that you would like the iPhone case for the next 5 years. I guarentee no one here, as much as they like the iPhone 4 case, will want it for 5 years. It would just get to stale, in a market where change occurs rapidly.
Look at the consumer laptop market, cases are redesigned every year. I know Apple has not changed their laptop case, but as I said above, this is the brand Apple is creating.