This service is way too costly for them to even make sense as a diversification item. They're paying $1,000,000 (=20,000 yearly paid subscriptions) per episode to well-known actors, and this to multiple actors, on top of production and service expenses. It truly doesn't make any sense.
Let’s look at some simple numbers then.
Assume a budget of 2 billion a year.
Assume each subscriber pays $50 a year.
You are looking at 40 million subscribers to break even.
And this is not including the extra revenue from selling additional streaming services via their TV app (which people now have a reason to use thanks to their own native streaming content).
I am willing to bet that Apple gets enough subscribers to break even within 3 years.