Apple's real goal isn't even the 30% from the Mac App Store. Their vision is to build a library of games that run on iPhone, iPad, Mac, and potentially Apple TV and Vision. You can connect a controller to all these devices, so any game would work (without clunky touch controls). That's why they're pushing Metal and will never adopt Vulkan. They want to make their ecosystem as strong as possible against competing ecosystems.
It's also why they've been pushing SwiftUI and Catalyst, why they don't care for web apps, why the Mac and iPad have gotten closer (they want each device just be a form factor that lets you access the same apps and files, though I expect they'll always keep the Mac as open as it is now), and why they made all their platforms adopt one design language. They probably ported Preview to iOS/iPad, and Home and Clock to the Mac, because they went through the Springboard/LaunchPad and asked themselves: "which of these apps could we bring to every OS?".
It's also why Google is dropping ChromeOS and switching everything to Android. One platform, one app ecosystem. They did it only to keep up with Apple. The tablet/desktop-ish side of their ecosystem lags far behind.
And it's why Valve is going all-in on Linux. Kickstarting an alternative ecosystem.