From 1 October, leaving Apple In-App Purchase drops a Small Business Program developer from 15% to 10% in the EU. On a 4.99 euro subscription, a processor's fixed per-transaction fee can eat most of that on its own - and your choice locks for 12 months.
Apple updated the Developer Program License Agreement on 18 August, and the new EU business terms go into effect on 1 October. The coverage I've read frames it as Apple finally opening up. That's true in a structural sense and mostly irrelevant to anyone running a small subscription app, because the interesting question isn't whether you can leave Apple's payment system. It's whether the arithmetic works. For most of us it doesn't, and it's worth being able to say why rather than just having a feeling about it.