Let's try a thought experiment, imagining what would happen if there was no prior opportunity rule.
1. Player takes possession, avoids tacklers, instigates play.
2. Player takes possession, is tackled correctly, makes an attempt at correct disposal, umpire immediately calls for a ball-up.
3. Player takes possession, is tackled correctly, makes no attempt at correct disposal or disposes incorrectly, umpire rewards tackler with a free kick.
4. Player takes possession, is tackled incorrectly, umpire pays free kick to player.
Does this make a messy situation simpler? No having to decide whether the player had prior opportunity or not. It takes one complicating factor out of the equation. Or am I oversimplifying?