You hit on the main point: removable from the device. Now you have a separate device to lug around that is at least 2/3 the size of a dedicated Micro-Four-Thirds camera body that will outperform an iPhone anyhow. And it cannot be deployed quickly. Price typically is not a prime concern for typical Apple consumers, so basically irrelevant.
Now it is an honest attempt to address my big complaint about taking cellphone pictures, which is the ergonomics of holding a flat phone for a photo (or video) is terrible. But the biggest cost is convenience. If you don't lug it with you, it cannot be used. And convenience it the entire selling point of using cellphone cameras.