Like read a dictionary, do you even know what the word means? Smartphone design has more or less stabilised and unless holographic hard light devices, like the ones we see in movies, are invented, I don’t think there is anything left to be done on the hardware side expect adding gimmicks every now and then.
The people who say this are the ones who idolize their phones, actually hold them and admire the colors, or whatever. The phone itself is a source of pleasure and entertainment for them. These are not normal people.
Apple developed a product that literally changed the world. There are now countless other products based on the iPhone. It is not possible to have any additional “innovation” with this product. It is done.
The next innovation from Apple will probably be the AR products they are developing if those have the capacity to change the way we communicate, change the way we research or consume information, or give us a useful new way to interact with others. This is not going to be the giant headset they are working on now, but it may be wherever that leads them.