I think adding abandon sanctions wouldn't go over too well with the community. The mode (while holding a different name) is still casual. It's meant to not be taken as seriously as an Unranked or Ranked, and exists as a way for players to play the game freely without worrying about elo, penalties or having to sweat for a W.
With that being said, that still doesn't diminish the frustration casual players face when their teammates leave, so this conversation is kind of difficult because it's nearly impossible to come up with a solution that pleases people who want to play QM and leave as they please, and players who feel games shouldn't allow people to ruin other player's experience by leaving them in an uneven/unfair matchup.