I don't know if the game does this now, but perhaps if a person leaves, then those who remain are given an increased amount of soldiers to balance it out? I'm honestly torn on whether punishment would be appropriate, as I personally think those who quit mid game are cowards, but I myself have experienced being kicked out of the game simply because of connection issues. But I think the aforementioned would work well in balancing out the game if it does come to that. I also think people quitting mid game is caused by lack of communication between team members, which leads to the team losing, which leads to frustration and abandonment.