No teamates remaining, still recieves a penalty.

That penalty is idiotic, and there are good reasons why most games don't have it.

Such as:

Yes, it's freaking annoying when teammates leave early, but you can't fix stupid, if they don't leave early, most of the time they are either worthless, they play solo, or they just troll, and you don't get much out of them anyway. So I don't care if people like that disconnect early, they are not helpful to begin with, so penalizing them later doesn't do anything for me during the round.

There are dozens and dozens of legit reasons why people leave iort discontent early. Punishing people for stuff like that helps nobody and just adds to the frustration.

Also if I am like really pissed at randos I am playing with, I will leave no matter what, penalty or not, and I am pretty sure quite few people feel the same at least once in a while.

At the end of the day the percentage of players that will not leave earlier because of the penalty, is probably so small that even having penalty system that is annoying for everyone by wasting player's time, is just not worth it.