A fine isn't even necessary: Apple's App Store business just ended. Between app sales and IAP it has a 30% tax. If an app developer swaps out for a Stripe portal they're paying 3% and probably at least doubling their own profit margin.
Only an idiot would still be selling apps through Apple's payments next week. The only way Apple will make any money at all on apps is if it drops it's fees to 10% or below.