Quite a bit hand-wavy! Only because SVG support is easily detected with the likes of Modernizr http://modernizr.com/. Background images with CSS are very simple to provide both SVG and a raster fallback graphic for. Image elements aren't as simple, but I'd suggest defaulting to SVG in both cases and replacing the source if support doesn't exist. That way the issue with eventually disappear alongside legacy browser usage.
As you said the benefits of high pixel densities are visually huge. A small and temporary JS hack is a price worth paying in my opinion.