Again, thats why you have two measurements. One is the unique decks, another is a measure of eveness. By using a measurement that is entagling them how are you determining whether the problem in a particular metagane is too few decks or not enough spread amoung decks?
You supplement the primary measure with secondary measures. But my goal, as clearly explained in the OP, was to find "a formula that could detect, and scale, a metagame that is more 'equal' and balanced, and one that is more 'unequal' and imbalanced."
This does that perfectly.