The default multi-monitor mode should be 'mirror'
Reason 1: Beginner friendly
Not everyone is a technical expert. Today, my niece brought a GNOME laptop to her university for a presentation. She failed because the projector screen only displays a background picture. She had no idea how to fix it because she hadn't encountered this problem before. Should GNOME chose 'Mirror' to be the default mode (like in Windows), she wouldn't have received an F.
Reason 2: Finding the primary monitor on first time boot
Imagine a set up with multiple monitors, install a brand new distro with GNOME on it, but the biggest monitor in the center doesn't display anything, as a result, the user spent quite a while to notice the UI in a smaller monitor.