Take New York Times. Last I checked, you can sign up online, but to cancel your subscription, you have to call and subject yourself to a slick sales pitch and all sorts of questions. It's inconvenient and borderline immoral.
One solution is, heh, the free hand / competition, but we clearly see that doesn't work all that well. A much simpler solution is to identify such behaviours and straight up outlaw that stuff. Which hasn't happened yet and probably won't for the foreseeable future.
Here's the thing: __The app store is in some ways like a government, and brings laws__. As long as that subscription were to run via the app store, you _CANNOT_ hide cancellation behind a phone number. Apple won't let you ship an app that works that way; in fact, apple does the billing.
I'm pretty sure that benefit pales in comparison to the problems that apple's sole ownership and onerous Judge Dredd-ian (judge, jury, _and_ executioner!) control over the fate of your apps on the apple app store bring.
But you paint the argument as zero cost to allowing alternate app stores, and __that is not true__.
Once alternate app stores or other enforced reduction in apple's abilities to enforce and police app store policy are in place, a bunch of apps WILL go sleazy on you where they wouldn't have otherwise.
NB: Of course, in practice, netflix etc. just do signups online, you get the same total lack of protection against sleazy stuff, and now the apps you download from the app store don't even tell you what you're supposed to do because apple won't let you explain that you need to sign up online and not in the app. Apple's solution / the current status quo sucks; probably sucks more than having alternate app stores available. All I'm saying is that having alternate app stores, whilst it brings a lot of upside, does bring downside, and that is (presumably) what the defenders of apple are trying to warn you about: Those downsides.