It’s bad UX because Command-Shift-5 brings up a UI that looks transient, and a user wouldn’t expect it to affect future invocations of Command-Shift-3 or 4. I’ve known multiple users who used Command-Shift-5 to take a single screenshot to their clipboard, and then lost all their subsequent Command-Shift-4 screenshots because they expect the screenshots to be saved to the filesystem not the clipboard.

The old `defaults` CLI command means almost no user would change the location of their screenshots so it would be consistent and predictable even when users switch between machines. Now a user has to use Command-Shift-5 first to configure the location before using Command-Shift-4.