Well maybe the team without a leaver can still keep their victory, but, you make a system to prevent people from abusing of leaving. SE already know when you leave a content, so they can for example increase the time while you can't queue after leaving. 1st time in a season is still 30 minutes, 2nd time is 24 hours, 3rd is 48 hours, 4th is a week and 5th is banned from ranked CC for the season. If you have more than 4 disconnection while you play CC in a season, well, don't play ranked until you get a decent connection... And if you do it on purpose like rage quiting, well, you'll deserve the ban.