You misunderstood. I don't expect screens to go away. There will always be iPads to pick up and sit back to read or browse through news, photos, videos, etc. There will also be devices on which we do our work on a screen. But our phones as communication devices, personal assistants and our link to the internet can be largely replaced by having it miniaturized on to our wrists [...]
My point is that Apple is not standing still taking the iPhone's success for granted. The people who think so today are of the kind who doubted Apple could keep the success of their company grounded on an mp3 player forever. Again, Apple has never stood still.
I see what you're getting at now, and I agree that real-time health monitoring seems most to be the "killer app" of a watch or some other future shift toward wearable computing. And, in a more general sense, that the efficiency and immediacy of an agent-heavy wearable at least has a chance of making them more prominent.
What I'm not convinced of is that the "small" size in the tiny-small-medium-large-huge (watch-phone-tablet-computer-TV) continuum of device screen sizes will fade away.
Looking at the way people use mobile computing devices today, and what seems realistic use cases for the foreseeable future, there seems to be a very definite gap between "tiny" (watch-sized) and "medium" (tablet sized) for walk around use.
Now, this could end up being a flexible display built into the sleeve of your coat, some sort of sci-fi looking wrist-wrap thing, an HUD built into glasses, or something that projects an image toward your retina. In much less realistic sci-fi-movie terms, this is the role of the hovering holographic screen that a person's watch or bracelet projects or the neural interface that brings up text and images in your field of vision.
But whatever form it takes, I'm pretty confident that there's going to be a substantial desire for something highly portable with a screen large enough to read longer-form text on, view photos or video on, and play games on. A watch-sized screen can't fill that niche no matter how powerful it is, and a tablet-sized device is too large to pocket when walking around without a bag.
Regardless, Apple is smart to be experimenting with wearables, and they're playing the long game, so I don't see the deficiencies of an Apple Watch today as any more disastrous than the deficiencies of a 1st gen iPhone or a 1st gen iPod.