This is ultimately a complex issue. Firstly, Quick Match is supposed to allow for players to feel that they can hop in a game if they don't think they have enough time to jump into a proper ranked match. There is no leaving penalty and is supposed to give a more relaxed way to play the game. This means that the system should allow for players to backfill in the event someone has to leave.
While it is annoying to be put in a match that is nearly over, the majority of the time it should be happening while a match is still mostly in progress. No backfill means players are put at a Xv5 situation, which is not entirely fair. Removing the ongoing matches could also increase queue time, which we want to minimize. I understand some players say they would rather take increased wait time over being thrown into ongoing matches.
Thanks for the feedback regardless!