FWIW, people usually like to support lots of payment options, particularly ones that already have the customer's payment details on file and that the customer likes using, so as long as you are OK with the realization that people who pay not through Apple are going to pay like 10-25% less (depending on any number of factors, as Apple is honestly offering non-0 value) than you (which I honestly doubt, but you need to remember that a key part of this isn't the functionality but the cost: a ton of the money you are paying is just going to Apple right now) I would be shocked if companies didn't throw in Apple's IAP as a (more expensive) option. Like: for my store (Cydia), I never required developers to use my payment options (as that would be a horrible and anit-competitive thing to do) but they did anyway because users liked them so much and they were easy for the developer and I actually solved problems they had (license transfer, some of the customer support, etc.).

Personally I would prefer to sign up to a subscription via PayPal, where I can easily manage the subscription though a web site instead opening the "Music" app (!?!) and navigating through that nightmare

I am confused, are you saying that the only way to manage your subscriptions in iOS is via the music app and not the App Store?