Awful Idea and it doesn't make any sense.
I can fully understand your situation, but punishing by leaving a match in a "no ranked" mode with no "e-sports league" or similiar doesn't make any sense and punish everyone.
There are several possibilities to clear the problem but a "penalty" or a "punishment" isn't the right way, and also if I remember correctly you gain a lose for leaving a match on your stats (please correct me if I am wrong).
1. One solution could be to implement a rework on conquest mode like Call of Duty Modern Warfare on their Ground War Mode did with the "DEFCON Countdown". This means as soon a team has all "flags" (in BF2042 all sectors) there should start a Countdown (1 minute or something like that) where the team has to capture at least 1 flag to show they are at least trying to capture and hold a point. This would give the losing team the preasure at least to try to capture a point and hold and if not well you don't have to play 20 minutes where the entire team is "outnumbered" or "camping" or doing "battlefield moments". For the winning team it would also be a favour because this rounds are often relaxing but also very boring. The next round/match should then entirely be new mixed or filled up. But this solution would be only for "Objective" based modes. ☹️
2. Another solution could be to investigate and gather intel why people actually leaving the lobby? What are they feared for? Is this because it's a waste of time? It's because they are afraid of their "K/D" (Kill/death ratio)? Or it's because the kids/wife/husband is calling for breakfast/lunch/dinner? Because you joined a lobby in a middle of a match and you don't want to play with these people who aren't able to out manouver the enemy team and at least try to capture another point because nearly everyone is trying their "challenge" or is "new" or just unable because the enemy team is to strong? Sometimes you played so good that you just got dropped on the loosing team will be punished with a "lost" for cause of teambalancing? Because you as player just completed your challenge doesn't want to play the entire round/match because you don't like the game mode? With that "knowledge" DICE should at least able to implement some sort of motivation system or karma system but this could probably also abused very easily.
3. Instead of also punish people why not the opposite effect... for a streak of not leaving matchs (except for disconnect errors (3 disconnect errors in a row should detect as suspisous), CE-Error (gameplay error's that crash your game) and leaving a round at the beginning of the round while the countdown is still running) should a player reward with extra progress on their "Battlepass" or something like that. This will give loyalty players a nice gift for their efforts to get over with the "bad" rounds.
There are to many scenarious for leaving a match... so a punishment system would make nearly only sense on a "ranked" mode.
Remember, Battlefield is no "E-Sport" .