24mm is a normal focal length for entry level macro lenses nowadays, yes. They don't just exist, they are very common in the entry level market. If you want to get those kinds of macro shots that's what you'll get.

If your goal for macro photography is to take a picture that is reasonably sharp at 12MP 2cm away with a magnification of less than 2, then yes, getting acceptable depth of field is a solved problem. Set your wide angle macro lens to F/22 and there you go.

If you have higher standards, then the problem is not fixed on DSLRs. But the iPhone doesn't do it either.

If you don't understand why using the CoC criteria for depth of field is incorrect on two cameras with vastly different sensor sizes, I can't help you. The only measure for depth of field that works across cameras with two different sensor sizes is the ratio of distance and aperture diameter, which determines the solid angle of light capture. You're the one that brought up physics, so actually look at the physics instead of using photographer's ready-made formulas without actually understanding them and where they break down.

As for the image that you replied to, it doesn't look any worse at all to the images in the post technically. If you look at the image of the lightning connector, it doesn't even have 2mm of depth of field at a pretty low actual resolution. You can say whatever you want as for the composition and artistic value, that's not what we're talking about.

I think you are right if you hold constant the CoC. In that case the DoF is proportional to the f number but inversely proportional to the square of the focal length (http://graphics.stanford.edu/courses/cs178/applets/dof.html).

However, for a smaller format, we arguably ought to reduce the CoC proportionally. And I think that reduction will end up canceling out one factor of f, bringing us back to the ratio of the focal length to the f stop (i.e. the absolute diameter of the aperture).