You are oversimpliying possible reasons why players leave early. There are multiple reasons, not just the one you've decalred. For example, maybe a player had a legitimate emergency, maybe their schedule is tight and they have other things to do ingame. Nonetheless your post covers the majority of leavers.

What i find most irritating are those that intentionally afk to get vote kicked to evade the 30min penalty. These players leave you guessing because asking to be kicked will often warrant a negative response. However the time lost in figuring out if someone is afk and getting a replacement in, has its own set of hindrances to the remaining raid of players. Players that demonstrate this kind of behavior need to man up and eat the 30min penalty.