I just joined a bit ago. First, I appreciate the forum and your participation. While I am now a "former pro", I now have more fun (!) and part of that fun is having Domke bags ... seriously. They work really well, and I am a BIG believer in canvas for all the reasons you stated. I have an F6 and F5X-B in sand always liked lighter colours due to it being less of a heat sink, and I don't think my F6 or F5X-B show dust badly. In fact I have a white car precisely because not only is it cooler in summer, but it hides dirt WAY better than a black or very dark car.
Another proponent of canvas was Bill Mason, the late dean of canoeing. Even though pack manufacturers gave him packs made with nylon and other synthetics to try, he kept using canvas as his personal choice due to its properties.
Plus, canvas just feels "right".