you're focusing on the "single window" part. the revenue-driven decision was choosing to make the OS more like iOS, locked down with an app store, rather than

you're focusing on the "single window" part. the revenue-driven decision was choosing to make the OS more like iOS, locked down with an app store, rather than macos, which allows third-party applications, browsing the filesystem, dropping into a terminal, etc. with built-in first-party support. instead of making a computer in an AR form-factor, they made an iPad in an AR form-factor.