In a team game, if a human player leaves (disconnects/crashes/quits/resigns), their spot should be taken over by AI at a moderate level. If all humans leave a team, the game would end with victory for the other team.
This would prevent disconnects and crashes from being a near-automatic loss. It would also compensate for the situations where a weak player quits a team at the first sign of trouble. It would encourage remaining players to fight on.
A moderate AI would contribute without being game-changing. Alternatively, the AI level could be based on the ELO of the lost human.