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.
Background images with CSS are very simple to provide both SVG and a raster fallback graphic for.
Oh, sure. But I was talking about just using SVG- for mobile devices with different pixel ratios falling back to raster graphics involves making a number of different size alternatives for each graphic. Not fun.
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.