I'm not sure this is true. I don't think that Sky Miles are an actual liability. Delta has 100% control over the availability of award seats and the redemption levels. Delta could cancel the entire program tomorrow if it wanted to, so its not a real liability.

I think that Delta just wants to give away less free seats because they will make more money selling them. I think they will continue to give away lots of Sky Miles as part of promotions or customer service apologies at the same time that they make them harder and harder to redeem.