Current HBO subscribers = 28M
  Revenue per subscriber  = $5

  Households that can support HBO Go = 5M
  Revenue per subscriber = $15
  Assume 100% of eligible households subscribe.

  (28M * -5 * x%) + (3M * 15 * 100%) = y
  x = 32.14%

For y to be positive (net gain for HBO), the chance that cable companies drop HBO needs to be less than 32%.

If we assume HBO Go adoption rates are the same as cable subscribers (26.6%), that means 800k subscribers.

  (28M * -5 * x%) + (800k * 15 * 100%) = y
  x = 8.57%

For HBO to have a net gain, the chance that cable companies drop HBO needs to be less than 8.5%.

Another way to put it, in order to risk $140M to gain $12M you need to be more than 91% sure.