Having one primary source of revenue vs having more than one is not the same thing as having a monopoly vs not having one. A monopoly is about the number of serious competitors you have, not the number of revenue steams you support or innovations you create.
To wit: there are companies with only one product without monopolies (e.g. pretty much every startup ever), and companies that constantly invent new products and do have monopolies (e.g. pharmaceutical companies).