The more I think about it, the more I honestly think that Apple will introduce a newly designed iPhone. Let me explain my reasonings.
1. The 3GS/3G argument is flawed imo, because the 3GS came out in June 2009 when the iPhone had no competition at the time. It wasn't till Jan 2010 that the Nexus One came out and starting giving Apple real competition. We are in a different time now where people are buying hardware for not just OS, but for hardware, too.
2. Upon asking my family and friends how they would feel if the new iPhone looked the same and was faster with a better camera they all said something along the lines of: "That's not cool, I want something different. If they look the same, why wouldn't I buy the iPhone 4 which is a great phone already"
3. Think about when people purchase a phone, they prob don't spend A LOT of time digging into the OS, they play around with the hardware itself. The OS is something that you appreciate AFTER you have the device for some time. Why do you think so many people buy Android phones and then after a few weeks of using it, realize it's not the greatest OS? You need time with an OS to appreciate it.
4. Imagine going into a NOISY Apple store and trying to demo Assistant feature to others. I don't think it'll be a great way to demonstrate that feature, no matter how awesome it is (I'm also assuming it's going to be badass) with the crowd and noise at Apple stores.
5. In 20 months of Android being serious (since the Nexus One) it has managed to moved from the number 5 spot to the number 1 spot. One of the big reasons is because they have different devices. I think Apple see's this and knows that there needs to be two CLEARLY different devices. One being a cheaper iPhone 4 for people who look at price, and one for people who are all about the latest and greatest.
I know this is long but real quickly imagine this scenario:
iPhone 5 new hardware, iOS5 with all it's new cool new features including Assistant, AND available on Sprint, Verizon, and AT&T. Also, a cheaper phone for everyone who cares about that lower tier price bracket.
If Apple does that, how many people will really walk into a store and NOT buy an iPhone? It'll be hard to pass up.