I want to have a single source of truth. It's confusing to have both DNGs and raw files in different places. Not all raws were converted to DNGs. When I see one raw file I can't immediately tell if it's a work file or not. I'm not concerned of files doubling in size. In total, they use up the same space, and one single file is not too big still (some people here shoot medium format, this is only 16Mpix).
Yes, it's very unfortunate that DNGs do not use sidecar files, though you can sort of force that to happen if you make your DNGs read-only. Adobe and FastRawViewer supports this somewhat well (though I hit a bug once that made me reimport the XMP manually for a few hundred images), but other software like Photo Mechanic, Capture One, etc, doesn't support this type of workflow at all, so in the end I abandoned it. My backup software is not bothered by everchanging files, it will only save the new blocks, and my files change rarely, usually just once, though it would still be better if we just had sidecar files.
It would be fine to keep the raws and DNGs separate, as long as I cataloged both, and automatically stacked the raws under the DNGs. Then there would be no confusion for me. But I can't do that manually with thousands of images and I don't know of an automated way to do it.