Earlier quoted context omitted.
But that's a too simple way to look at it. I am not trying to defend the 30%, but just saying 'their own apps do not have to pay 30%, so they can easily replace any app with their own' doesn't go far enough. They got two options: a) make a 30% on $10/month from Spotify -> $3 in Apple's bag for no work or effort at all. Almost pure profit. b) sell their own music solution for $10/month (INSTEAD of that customer going…
$3 in Apple's bag for no work or effort at all I agree with most of what you said, but come on. Running (one of?) the biggest software distribution and payment processing systems on the planet is hardly "no work or effort at all". This is the same sort of rhetoric that calls dropbox a weekend project.
There's credit card costs etc associated with that as well. Doubt it's $3/user per month though and the distribution costs are minimal (only downloads of app updates) compared to the streaming costs (which could be multiple GBs every month).