Might want to try the Overwatch approach.
In Casual play:
You’re free to quit, but within say 10-20 matches, if you left X amount of times early, you will receive XP/Key 75% penalty until your stayed in game percentage goes back up.
Of course the main problem is that the game no longer allows drop in replacements.
If a person leaves the team the other players are free to leave the match after X amount of time.
Without a loss in their rankings, just a lost match on the tally sheet.
The person that left gets a loss, plus goes down in ranking. I forget if they give them extra loss of ranking for leaving, or just regular amount.