Not offering in-app signups doesn't seem to make Spotify less dominant. I'm in the Netherlands, almost everybody I know has a Spotify subscription, I know just one guy using Apple music.

The 30% fee also drops to 15% after one year, and there are companies that negotiated lower fees. Also, 'doing it yourself' won't be free, you still need some party to do payment processing, customer service and returns which also can come close to that 15%.

The argument you need to make your own phone seems a bit far-fetched. There are multiple music apps making money on iOS.