MF market is a small one, but it's a professional one - where users can spend a lot if they can see a real benefit in a product. MF is all about quality for demanding users in fields like fashion, advertising, arts, etc - images that may be shown at much larger sizes than the A4/A3 format, or reproduced carefully in very high quality prints.
A "cheap" MF line would find almost no customers, MF cameras are bulkier and less comfortable to use, and their lenses as well, DSLRs are better for most users, while professionals won't accept less quality.
If Canon will ever decide to target that market, of course the only way to get its market share is to deliver state-of-the-art sensors (60M+), cameras, and lenses. Not a simple jump for a company with little or no experience in the field, IIRC Nikon produced lenses for MF and LF cameras, while Canon never did.