We're still at the point where most of the serious icon artists
create in SVG but have to deploy in multiple PNG versions per icon
due to the runtime limitations of current display engines.
I don't think it's only because of the limitations. Even with perfect SVG
support I would still deploy in PNG, because it's faster to load. And with
multiple sizes looking different, there's little reason not to deploy PNG.
The solution is not for Inkscape to behave differently than the rest of the
desktop, but rather to improve librsvg, or modularize Inkscape enough that
its renderer could replace librsvg (I doubt if that's possible, because
librsvg is SAX-based).