"Appetite is essentially insatiable, and where it operates as a criterion of both action and enjoyment (that is, everywhere in the Western world since the sixteenth century) it will infallibly discover congenial agencies (mechanical and political) of expression"

Marshall McLuhan is recommending that the desire for usage (appetite) in Western societies is limitless which this yearning will inevitably cause the production of systems or systems (both mechanical and political) that enable such usage. Essentially, he is arguing that the Western world has ended up being progressively driven by consumerism and this drive is insatiable, causing the development of structures that allow for ongoing usage.