When people provide icons in many physical sizes, the icons generally aren't mere scaling up and down, because even if you can hint so the lines themselves don't blur the details will get so small the whole icon becomes a mess. Apple even has a note on this in their HIG:
> If an icon’s content or shape is overly complex, the details can become confusing and may appear blurry at smaller sizes.
note that this is not about logical size and retina displays, it's about physical size and the viewer's eye's power of resolution.
That sounds more like a problem than a solution to me.