Dust is in the air. Lenses lenses that extend while zooming or focusing pull air into the lens. As long as they pull air into the lens, you need a filter to catch the dust. A shroud does little if anything, since it does not filter dust. You could seal them air tight, but then, you could not zoom or focus.
Perhaps if you added a air filter, it would help, but you don't want to force the lens to pull air in from the rear, dust in the rear is a serious issue, it means nothing in the front.