That was not at all obvious to anyone in 2008. It's a pretty typical margin from the console industry, which was the main point of reference at the time. Apple spent several billion building the App Store infrastructure, the SDK and setting up the review and payments system and it took years for App Store revenue to catch up with the sunk investment and expenses. Also practically everyone in the industry at the time was saying Microsoft and Google would imminently wipe Apple out of the mobile market.
This is a tough one, I think 30% made perfect sense in 2008 but does seem steep now. Apple makes huge profits on the App Store, but it mostly seems to have happened by accident, their original strategy seems to have genuinely been to just break even and maybe make a modest margin, with the store mainly just being a competitive advantage. The huge success and profits have been a windfall.
However that all happened and we are where we are. I don't object to Apple's app store margins or IAP charges, it's their product, their rules.
I do think banning developers from informing users of how to get subscriptions and such outside the store is foolish. That's clear overreach. I see why they do it, otherwise subscription services can cut Apple out and free-ride, but they probably just have to take the hit.