I can easily recommend what SE can do here, sense this seems to be a problem with end dungeons. Why don't they just threat end game dungeons like Guildleves. You get a bonus if you complete the whole dungeon without anyone dying (cuts out speed runners), and if your whole party completes the dungeons fast. The person that leaves forfeits his/her bonus, and the people who stay keep it, and the next person that joins get the original reward, but not the bonus.