Yes, this Argument also holds merit and is a huge issue regarding the music services.
It was brought up when Apple music launched and still has some viability. if Apple charges 9.99, Spotify charges 9.99, and GPM charges 9.99 for their services. Apple will automatically take home 15% more from their own service, while ALSO receiving 15% from both GPM and Spotify.
this is almost anti-trust territory where Apple's own services automatically get a competitive advantage. However, being that Apple isn't a monopoly, I'm of the opinion that if competitors really don't want to pay Apple, they should pull their apps.
How well would Apple's iOS platforms sell if suddenly there was no Netflix, No Spotify, No Hulu, No Youtube, no... well, you get the picture.
Apple is no longer the #1 place to develop for. in 2018, we've seen for the first time Android receive more new programs and apps than Apple. Apple cannot continue to prioritize their own monetisation if suddenly the services that people want are no longer available.