When you do a full blown variant there is of course a merge capability, and you obviously need a variant when you have highly differentiated items of various sizes, etc. So the variant type of product is not the problem since variants have the merge button. However if you have a "semi-variant" where you combine various similar looking items using different model files there is no merge capability. For example, you have had two distinct models with different wall thicknesses in your shop with photos, renders, and comments for quite some time. But now you wish to combine them into one product. The thin walled versions might be for FUD and the thicker walled versions might be for metallic plastic, steel options, or sandstone options. For this case there is no merge button and there's no reason to go to a variant option because there's not anything that would qualify as a variant for such a product. I'd say if multiple model files are being used in a product it should cause the merge button to appear on the product editing page.