There was a missing link, as Hockney mentioned, at the end of the 14th century between those paintings that contained little depth of field and a general flatness to those that inexplicably were of better composition and quality in the early part of the 15th century. The use of the mirror as a projector makes that transition more understandable, as well as, explaining how photography eventually came to be.
Like Grant's analogy with sports, you can almost use any number of fields to compare with that of art. What I mean by that is, there are many techniques and tools utilized in a various fields of interest. Techniques or tools don't necessarily change the validity or quality of the product. It's just simply a means to a final outcome.