You're assuming that billions of Apple customers are actually eager to pay for News subscription services. You are likely wrong.
No. The assumption is that getting content in front of that many eyeballs will result in an increase in revenue that makes it worthwhile, that otherwise would not have been generated.
It is a logical fallacy to suggest that X customers need to subscribe.
All that needs to happen is to change 'current subscription revenue' to 'current subscription revenue + X' Where X is the minimum amount the publishers feel would make participating be worth their while.