If Apple is so bad at this that they have to charge 30%, they should have failed in the free market to a competitor that can do the same or better for 3%. However, Apple has prevented that, not by being better or cheaper, but by implementing DRM that locks users out from having a choice (and the market as a whole ended up being a duopoly with cartel-like pricing).

Whether Apple can be cheaper isn't really the point (they should be, digital services are a very high margin business). It's that they're anti-competitive to the point that the market for paid apps and in-app payments became inefficient (in a financial sense).