I want to change the battery on my phone

Is that so much to ask? Can Apple just wait for a little bit on the new iPhone, and instead just release the iPhone 4 again with a cheap interchangeable battery?

I don't mean like iFixIt style interchangeable, I mean take off the back and swap it with another one.  

Good design is being realistic

 If you release a phone with 5 hours of battery life, you need to release it with an interchangeable battery. You don't see the GoPro people saying "Live with it, you only get 1.5 hours of battery life." They make it easy to swap out the battery for a new one. They said: "Yeah, it is hard to make a small package with good battery life, so here is an alternative." Of course, $40 is a lot for an extra battery, but at least they provide the option.