I would disagree with "average casual user" or maybe I think about them differently. For me average casual user now is completely fine with Linux distribution like Mint or Ubuntu or similar (or maybe chrome os). And with that they are little bit safer online as they are usual targets and victims of malicious software etc.

I think casual users are type of people completely happy with internet browser, media player, image viewer and just basic software ... They are usually satisfied with regular Android phone Wich is enough for everything they do in computer space.

This type of users (like my mother and other members of family) are fine with major Linux distros. They don't care about OS they use, it means nothing to them.

This is where I draw the line when suggesting Linux to people. If they don't know and don't care ..Linux is usually fine. If they are aware of what type of os they use or even what version ( talking about Windows) I will suggest Linux only if they are open to it and I'm willing to help and recommend some software alternatives.