Apple should drop or tier their pricing so that a $10 subscription is paying say 5% to them. Developers are happy. Anyone raking it in pays more, a progressive system that ensures small companies can innovate.
Is this bad for Apple? In the long-term, they get more innovation and a better user experience. Financially, they lose some income on a subset of existing transactions and gain income from transactions they wouldn't have processed, as apps switch to native dev, and the market sizes increase as incentives change.
At the most they could lose a few billion in revenue a year (very unlikely) and gain $0. That's the upper bound. But how many more subscriptions would go through the store if apps like Spotify had signup via the app? I'd guess hundreds of millions of dollars at least, could be much larger in the long-term.
Apple are losing the argument, even the best possible cut isn't worth the reputational damage and harm to the user experience, they should restructure the pricing so they save face and potentially benefit, or at least limit the damage.
The writing is on the wall at this stage, and they will act or be forced to act.