Apple cheerily breaks functionality for end users when they decide to remove or replace APIs on every major release of macOS. This tends to hit power users especially hard, as they often use tools that extend Apple's narrow facilities for things like handling keyboard input, managing windows, or controlling applications' audio volume through third-party tools that rely on internals of macOS that are not supposed to be public-facing, and those get broken more often. If your favorite tool relies on an API that gets pulled and doesn't (or can't) get rewritten when the new macOS release is in beta, you're SOL.

Homebrew breaking on macOS upgrade is also due to one thing that's fundamental to Homebrew's design, which is that it tries to rely as much as possible on that same external base system which Apple provides but destructively revises whenever it sees fit. (This design choice is apparently the author of Homebrew made because relative independence from the base system meant too much recompiling for him on Homebrew's competitors at the time.)

Package managers are used to install a hell of a lot more than bash. The idea that they wouldn't be necessary on macOS if only no popular tools were licensed under the GPLv3 is laughable.