That makes sense, I guess. I've never owned a canvas bag so I don't know how that would work. Of course that would be different with an epi but I've not seen a slouchy epi, fortunately.I think an insert would help even the slouchiest Speedy, since it's canvas and not leather! The canvas doesn't stretch the way leather does so structure should be ready to add back in with an insert. My oldest Speedy is about 10 years old and I don't see why not!
I'm cliche enough to want a bag from my birth year, so there's that