It’s tough… They’re leaving because they’re not having fun.
Imposing penalties for leaving works great for ranked matchmaking, but for casual, it’s a pretty awful idea I think.
For the penalty to work, you need to make the penalty more painful and severe than it is boring and frustrating to stay in a game that you aren’t enjoying. You’d need to make the penalty pretty harsh.
This sucks for casual play. If people are forced to choose between two awful options, leaving and getting a penalty vs staying and being bored/frustrated, they’re realistically just going to say “fuck it” and take the third option: stop playing. That sorta penalty system would also affect people who crash, timeout, or just lose internet.
There’s no simple answer here. :-/