While I think most applications and most designers would benefit from vectors instead of PNGs. You seem to forget that there is a perfectly valid reason to still use PNGs for app icons. It’s due to the fact screens still use pixels. When you scale down an image to half its size, a line that was previously 1pt wide would appear blurry which is why in many cases app icons keep lines at 1pt no matter the size of the icon. It’s very common practice to make icons look different at different sizes because simply scaling them down doesn’t do the job.
To see examples of this you need look no further than your operating system’s folder icons, the 16x16 pixels are highly optimized to be distinguishable at a very small scale while the 1024x1024 icons offer great detail.
I think that the vector graphic standards need to be adapted to offer non-scalable components. For instance being able to set a stroke to a fixed pixel/point size—while still being able to resize the overall art or shapes—would be the solution to this problem in almost all cases.