It's 100% of iPhone users.“Developers have to be on iOS to reach the ~30% of the European market that represent iOS users, but musicians don’t have to be on Spotify to reach the ~65% of the European market that represent Spotify users” is a very interesting take.
I suspect not many musicians have the clout to get casual fans to switch streaming providers.
If Spotify changed terms to the disadvantage of musicians it would be easy for musicians to organize a mass exodus. That keeps Spotify somewhat in check with their pricing.
If Apple raises fees, there is nothing you can do as a publisher because there is only one store.