Size! I'm very happy with my light-weight rebel body, and light EF-S 10-24, 18-55, 55-250 lenses. But I'm also happy to put the 100-400 L II, or a EF macro lens on that body. I'd never buy a 100-400 and matching RF body, just to be able to use a 100-400 lens. And I doubt Canon will create their whole lens lineup in duplicate mounts for RF and M.Let's face it: APS-C was invented in a time that full frame sensors were very expensive. Now that that problem has been solved, who needs APS-C?