Except that it’s not...not anymore. It’s primarily a computer that can take phone calls. How many people are still taking phone calls instead of texting, messaging, emailing, or connecting in other ways via social networks? How long is the Phone app used compared to Safari on your phone’s battery settings? For how many people is Phone at the bottom of that list? On mine it is.

Finally, your point is still completely irrelevant to why it needs to take over the entirety of the screen. It could still primarily be a phone without being intrusive when you’re doing “secondary” tasks.