Optimizations are mostly needed for very small icons when there's only a
limited amount of pixels to use, however, more details can be added at larger
sizes (up to 512x512 pixels). Vector icons are NOT the type of
one-size-fits-all. They look good within a range of sizes they were designed
for. Desktop icons fit into the general/mid-range so they're not really
affected by this.
Here's a real-life example of an icon optimized for different sizes (icon
comes from trisquel-icon-theme):
And this is a visual representation of what I mean: