Maybe we need a dumbed down version 3 of SVG where the browser knows it is not to do anything that requires fetching a URL, to make the image as harmless as a JPG.

This version 3 could have the version number changed to 2 in order to do cool SVG things, so full-fat SVG as version 2 is now. But you could just flip to 2 to a 3 on upload, so any embedded URLs are harmless.

This could be useful for the creator too, as it is helpful to have layers of source images in bitmap format to work with, and you can easily export such things accidentally.