A rhizome will only flower once. It's increases are "next years bloom". So I call the iris a "Cycle Perennial". The "mother" rhizome I do not believe is "food" for the babies/increases as the new increase put out their own roots. The mother would also hold the increases in place and combat frost heaving; but you could discard the mother as described. I would still think you would need to dig up the clump anyway, but maybe at a later date than if you did not cut out the mother rhizome. My 2 cents.
