Personally, as an app developer, I think Spotify is taking the right stance here.
I believe that Apple should take some cuts but not as high as 30%. I believe for some categories like microtransaction based apps Apple should take maybe 5%. Plus, I think that if Apple has a competing service, then it should waive the tax altogether. It's only fair.
If the only function of the App Store (or Play Store) is to provide hosting and some quality control then I don't see why apps can't be hosted on a secure website from the vendor - as far as I'm aware, Apple requires you have some sort of website anyway.
At least Google allows installing apps without the Play Store, perhaps it's time Apple permitted something similar with its apps? This could solve this whole problem and have other positive side effects such as people being less likely to jailbreak their iPhones.