Because of the numbers in both pots, you can't compare percentage. A 15% drop in ESPN viewership equates to 350,000. A 5% drop in cable/satellite subscribers equates to a little less than 5 million. The pots are totally different in size, and only a small number of individuals are members of both pots. ESPN has lost only a little bit in overall ad revenue, but that 5 million subscriber drop accounts for close to half a billion in lost annual revenue.
It's not that the declining viewership isn't important, because it certainly is, but where the drop is occurring does matter. College football is not losing viewers. College basketball is not losing viewers. The NBA is not losing viewers. Monday Night Football is losing viewers, and given that they spend $1.9 billion a year on MNF, that's a big deal for ESPN.
That 1.9 billion number is staggering in the light of the subscriber losses. In just two years, ESPN has lost a quarter of what they need in order to pay for 17 weeks of MNF. And it doesn't make a bit of difference if those subscribers watched MNF or not; either way the half a billion is still gone.