You're right - you don't have a choice if you support scummy business practices with Apple. When you use an iProduct, Apple demands it. Every dime you spend on the App Store gets taxed by the richest company on Earth.
So companies "will do everything they can to screw over the customer". Apparently excluding Apple for some reason who can never do wrong or suddenly start demanding apps charge subscriptions instead of a flat fee in the first place, but let's continue.
Companies that have their own infrastructure to support their own payment systems will switch to it, but have to create a justification for consumers to switch as Apple isn't going to do that work for them. So maybe they cut the fee, maybe they throw in a bonus whatever. Some consumers switch, finding this new method acceptable.
Then there will be other consumers, like you, who value Apple's system too highly to switch. If the above hypothetical company does not support both Apple and their own systems, this group of consumers will opt not to continue subscribing.
Now tell me, how is this so bad? How are you unable to avoid scummy business practices?
No it doesn't. If you use Apple IAP (or any 3'rd party IAP) you cancel there and the 2'nd party can't do anything about it. What you are claiming is just a lie, a strawman.
The difference being discussed is between <Apple enforcing Apple IAP as one of the options for IAP> and <Apple enforcing Apple IAP as the ONLY option and banning any communication on the subject>.
I am talking about payment services. Most companies will do everything they can to screw over the consumer. Especially once they have you in a subscription.
As a consumer I would rather these companies be forced to not do shady practices and have to conform to a uniform way of purchases and subscriptions.
I have been in too many meetings about "User Retention", if given the chance very few companies will choose to loose the ability to bash the user over the head with "are you sure you want to cancel" or "please call to cancel".
Apple forces it.
Edit:
Put Another Way: Apple giving developer the choice of wether or not to use their system removes MY choice as a consumer to avoid scummy business practices (Which are often not on display until you no longer want to do business with them).