Who cares if you prefer IAP? Use IAP then. The discussion is not that IAP is bad, the discussion is that it's the only option, hence no option. You can't even talk of other options.
I can't, because--and this is key--Spotify refuses to implement IAP! To implement IAP would drastically undermine their case, eliminating huge swathes of the linked page.
Yes, it's dumb that they're not allowed to link to the website for people to purchase directly. I would prefer that they handle it like "login with Apple," where you can support others, but must support Apple alongside. I would always choose IAP, others could click and go through the extra steps to buy directly from Spotify. But it also isn't that hard to deal with. Amazon does it for ebooks, and people still buy ebooks from Amazon by going to the website.
Spotify is throwing a fit because they don't want to even allow IAP. They want the advantages of being on iOS with zero consideration of Apple or iOS users. Apple is going too far, but if I have to choose, I side with Apple here, because Spotify is throwing a dishonest tantrum.
Is that better?
I will never use a purchase method on iOS that isn't IAP. Never, and I'm not alone.