You're suggesting a simple view where you look at the average amount people who subscribe to this (e.g. $10/month) service would have otherwise paid, and that if that number is less than $10/month the devs (/musicians) win. This is certainly true.

However, there's the additional benefit that not every user that subscribes will entirely drop their purchasing habits. Perhaps less often, but some subscribers will buy apps (/music) on top of their subscription.