If it's another on-demand style content provider like Netflix, Hulu, Amazon Prime Video it has a very high likelihood of failing. This is an industry model that is already quite mature with large customer bases and massive content deals in place. There isn't a lot of room for a new player here. I doubt that people are going to be willing to pay what Apple will charge for the content that they will bring to the table. I doubt even more that people will be willing to switch from their current services. They'd need to target bringing the ENTIRE iTunes catalog of Movies and TV plus original content to streaming with a price no higher than about 14.99 to make anyone even think about it.
On the other hand, if they plan to offer live TV and compete with PS Vue, DirecTV Now, YouTube TV, Hulu Live, Philo, etc they could stand to succeed in a big way. This model is in its very infancy and no single provider has found exactly the right mix of content (channels), price, and features. There is a ton of room for growth and this area is just begging for someone to swoop in and do it right.