Most respectfully and deferentially, I don't think "more things available makes price goes down" is a platitude or textbook or academic thing!

I must confess, the fact this Fat Tony logic and not some theory invented in an academic textbook ivory-tower divorced-from-reality impoverished-intellectual safe haven is why I cannot provide a proof: the note about the margin was an attempt to inject some levity, via a reference to Fermat!