Declining sales of high-end cameras and lenses are raising an alarming question for companies like Canon Inc. and Nikon Corp.: Could the proliferation of camera-enabled, app-heavy smartphones be crushing not only the simple point-and-shoot, but premium models as well?

This year, shipments of what's called "interchangeable-lens cameras"—high-end models that let users swap out different lenses—are diving suddenly after years of robust growth. Most of those are digital single-lens reflex, or DSLR, cameras—the bulky models...