I agree with OP. Leaving is a problem, at least in my experience, and the penalty is not harsh enough. The reason I say that, is not only do people leave mid-match (often after throwing out some toxic chat), but people will leave matches with 4 or 5 seconds left.
Meaning, the penalty for leaving is not enough to get people to simply wait 10 seconds longer for the ‘Defeated’ or ‘Victory’ screen to appear.