They should implement a system where people with endorsement level 4+ dont get put in backfill games. Itd actually make the endorsement system useful beyond knowing the player is possibly a nice person/good teammate.
They should have a point timer start when the match begins and have the points accumulate over the course of the match and if you leave the match cause you get salty you take the hit to your endorsement level based on how long youve been in the match. That way, hopefully, if someone leaves a match they think to do it early on or take a big hit to their endorsement level.
But they would also need to give 5 votes so you could endorse your entire team for playing well if you want or just the 2 supports or whatever.
They should also give all or 2/3 the experience the leaver accumulated during the match given to the backfill player and give the backfill player a soft boost to match making after the backfill game. Since they probably waited for 5-10mins (possibly longer for those of you above diamond) for the match and get tossed into the match in progress.
For the players that get disconnected and get hit with these penalties, it sucks, but its to improve the community so take it on the chin and call and yell at your internet provider.