Compared to? All taxes mean fewer transactions – and purchases that never happen don’t generate revenue. This is not easily measurable. Most SaaS or similar services probably go through other devices or email to handle transactions.
Apples payments and subscriptions are excellent products but how much goes through there of say revenue for streaming services like Spotify and Netflix? Is that even supported? As an iOS user, I would never dream of trusting that the IAP prices are a good deal, and especially if I want to support the company I would sign up on their website.
I’ve said before that even if they were allowing (or better, forced to allow) competition, people would still pay a premium, maybe 10%, to have all subscriptions in one place with one-click unsubscribe. They simply don’t believe their own product can stand on its own without crutches, for some bean counter reason.