But they're only standard because the credit cards asserted these fees, we got used to them, and now they seem obvious, right? (I remember plenty of merchants who used not to accept credit cards to avoid the fees, but, whether by bank pressure or just the weight of expectation, there aren't many of those any more.)

It surely won't be too long until these fees are 'standard' too, in the sense that they're totally usual and customary, whether or not they're reasonable.

It's a difference between small merchants and large merchants. Small merchants still feel free to refuse cards or charge higher prices when you use them. Large merchants aren't free to refuse (too much of a customer loss) or to charge (against the credit company terms).

A tax for existence on a platform is just rent seeking and increases prices for everyone but Apple. That's not desirable and likely illegal.

The worst case scenario is Spotify-cation of services where Apple used the App Store to identify popular services, counters with their own, and then charges extortionate fees such that they cannot compete with Apple's native services. I would be concerned if we start seeing an Apple Brokerage.

Apple Pay doesn't have a direct competitor in Venmo since Venmo is about payment processing between people who don't normally deal with credit card processing, and cross platform functionality is the biggest value prop. Apple Pay isnt a threat unless it works on all platforms equally.