The problem with zero trust and only sending what the player can actually see is lag. For example, sure, you could eliminate wall hacks by only sending position

The problem with zero trust and only sending what the player can actually see is lag. For example, sure, you could eliminate wall hacks by only sending positions of players that you have line of sight of, but that essentially eliminates any client-side movement prediction or other lag compensation and means you have to send each frames data quickly enough. This may work for some games that are less lag sensitive and as internet speeds improve, but its not currently a solution for most players.