With GoogleTV being phased out and AndroidTV taking its place, how long will our GoogleTV app be viable?

GoogleTV will still remain viable for at least 2 years as the enormous variety and number of devices already in use will still be supported according to Google. That said, they will start promoting AndroidTV moving forward and manufacturers will begin to implement AndroidTV on their devices moving forward. This will slowly decrease the number of GoogleTV supported devices and increase the number on the AndroidTV side.