Prior to the App Store, boxed retail software generally had a 50% cut.

Also, you can call it a duopoly, but Apple didn’t leverage market share into this 30% cut; it started when they were closer to 1% of the phone market, and the policy has only ever gotten cheaper (second year of subscriptions and small business program) and more permissive. They offered a closed platform and competed their way to the top based on the product. Developers are given the chance to sell to a huge market of high-end hardware with aggressive consumer uptake of software updates, making it a very attractive platform. Apple wants a commission. It’s not exactly outrageous of them.