When the iPhone App Store first launched, Steve Jobs claimed[0] the 30% was to cover the cost of certifying software as functional, well-designed, and nonmalicious. Part of it was an ego thing too: he didn't want people fucking up apps and making his pet project look bad, so early App Review focused on a lot of UI polish things in order to make people think iPhone software was just inherently better than Android.
Even a few years in there's already evidence that Apple was entirely aware of how much of a cash cow owning the distribution market for your apps is. There's an internal letter asking about reducing the percentage because someone was worried about the Chrome Web Store (?) eating their lunch. Today, App Review is far too inadequate for the level of software submissions Apple gets, and they regularly let garbage onto the store that's specifically supposed to be curated.
I occasionally hear people complain about how Tim Cook "ruined the company" and that Jobs would never do the kind of control freak shit that he literally pioneered and is literally the selling proposition of the Mac all the way back in 1984. The only thing Tim Cook did was scale the business from "luxury compute" to it's inevitable conclusion as a monopolistic nightmare. The way that the App Store business game is played is specifically that you don't keep spending all your money on better app review. Once you have users and developers mutually hooked on one another, you siphon money out of them for your other projects (or your shareholders).
At one point, you were paying a premium for a better App Store, but not anymore. The business relationship just doesn't work out that way long-term.
[0] I personally think this belief was genuine at first.
To add more evidence to your point: SJ loved wall gardens and consistently fought against extensibility. The Apple II only got extension slots because the other Steve insisted. All of the compact Macs have very limited to no extensibility.
It's so ironic that Apple was pushing the (open) Web apps in the early days of the iPhone (out of necessity of course).
Jobs loved excellent user experiences, and, rightly or wrongly, saw walled gardens as an important part of providing them. Sometimes.
The counterexample is the iPod, with its advertising slogan "Rip. Mix. Burn.". The first iPod used Firewire and was Mac-only, every edition since then used entirely industry-standard technology, USB and MP3. The value proposition was, as the slogan illustrates, easily taking your CDs and putting the music on the iPod. That too was in pursuit of an excellent user experience.
Later, Jobs fought the entire music industry for the right to buy digital music, not just rent it. And won.
Even a few years in there's already evidence that Apple was entirely aware of how much of a cash cow owning the distribution market for your apps is. There's an internal letter asking about reducing the percentage because someone was worried about the Chrome Web Store (?) eating their lunch. Today, App Review is far too inadequate for the level of software submissions Apple gets, and they regularly let garbage onto the store that's specifically supposed to be curated.
I occasionally hear people complain about how Tim Cook "ruined the company" and that Jobs would never do the kind of control freak shit that he literally pioneered and is literally the selling proposition of the Mac all the way back in 1984. The only thing Tim Cook did was scale the business from "luxury compute" to it's inevitable conclusion as a monopolistic nightmare. The way that the App Store business game is played is specifically that you don't keep spending all your money on better app review. Once you have users and developers mutually hooked on one another, you siphon money out of them for your other projects (or your shareholders).
At one point, you were paying a premium for a better App Store, but not anymore. The business relationship just doesn't work out that way long-term.
[0] I personally think this belief was genuine at first.