The ability to simply cancel any subscription without clicking through increasingly desperate "Don't go!!!" nag screens is easily worth an extra 30% for me, too.
This means that competition is working, because you would choose apple's IAP, over the third party, even if it was more expensive.
Aren't you heavily assuming these developers would even bother integrating apple's IAP? This ruling will allow the developers to completely go around apple and never use apple's IAP at all.