There's already PlayStation Vue, while they have not launched an Apple TV app yet the login works for every Apple TV app. Comcast its trialling a service, Hulu is planning to launch a cable service, and at&t/Directv are also planning a service.
I certainly wouldn't mind subscribing to a streaming service from a traditional cable/satellite service, or Playstation Vue. But I'd want to be able to pick my channels. Otherwise it's just cable/satellite over the internet.