It’s not the phone part you have to design around, it’s obviously all the other stuff, especially – really, really, especially – if your argument basically boils down to “Oh, but it’s fine for making phone calls!” I mean, yeah, you can easily argue making the situation better for phone calls in your design since they still play some role, but the argument you made just doesn’t fly at all. It makes no sense.
It makes no sense to you. But the world is a lot larger. If there is an interruption in data services most people wouldn't even notice. But when you can't make or receive calls that's major and in extreme cases will lead to loss of life. Apps are nice-to-have, phone calls are a must.
Try marketing a smart phone that can't make phone calls and see how it goes.
> You just cannot design them with that in mind as something to optimise for.
Every smartphone has been primarly a phone, secondary a camera and thirdly something that runs apps.
It's essentially a computer with a telephony interface but that telephony bit is absolutely essential and not an afterthought.