Nope.
Here is what you are missing. It doesn't matter to Blizz where that money is coming from. Either you pay $15/mo via subscription or you give gold to someone who paid $20/mo on your behalf.
Blizz actually makes more money on the deal. The fact that someone paid $15 for his sub and another $20 for to get your gold while you paid $0 for the month is not relevant to Blizz...they walk away with $35 for $30 worth of goods.
So, quite the opposite of what you think. Blizz stockholders would rather the game get rid of paid subscriptions, and get their money entirely based on the $20 gold transactions, in which case every subscription would go from $15/mo (or less for those that pay multi-month) to $20/mo from the financial perspective.
However, the next step in economics is the system you see today. In reality, some people have more time than money, and others have more money than time. This current arrangement allows Blizz to maximize their profits by cashing in on both groups of people.