I definitely think the current leaver and backfill system is a bit messy at the moment. It's very slightly better than it was as your stats are no longer affected if you lose a backfill match. But it is still incredibly frustrating and tedious to sit in what can sometimes be a 5-10 minute queue, just to load into an instant defeat screen and have to re-queue again.
If you look at it from another perspective, you wouldn't want your team to be a player down for any length of time because it's such an enormous disadvantage. So backfills are necessary to make it less frustrating for those in-game who have to deal with a leaver. Especially for those who are already winning and someone leaves. But there are also a lot of scenarios where there is every chance of winning and making a grand comeback if someone can join quickly.
It's those games that are completely unwinnable, where you barely have a chance to pick a character before you get slapped with a defeat screen, that are the most frustrating. Perhaps there is a system that could be introduced where if the game passes a certain threshold of points, or if there is a certain difference in points, then backfill is denied. But how can the game accurately and consistently determine which games are unwinnable and which games are not? It's impossible. I have joined games in the past - not many though - where my team has been 400-500 points behind in Objective Control, and we've brought it back to win. Likewise, it only takes one team fight to turn the tide in a Power Cells match, regardless of the score. It just doesn't happen nearly as much as an insta-loss though.
I have suggested a "Vote to Forfeit" system in the past that may reduce the amount of backfill games we have to suffer. In those scenarios where a team is down a player and down a lot of points, a poll to forfeit the match could appear. To prevent abuse of the system, it could only appear in those certain thresholds I just mentioned above, when the game detects that "all might be lost". Then, it's up to the players to decide whether they believe they can still win if they get a backfilled player or not.
There also needs to be adjustments to the leaver penalty. I have made an extensive post in the past criticising the need for any sort of harsh leaver penalty. I don't think the penalty in place right now - where if you quit 3 matches, you lose rewards and are put into a "leavers queue" - is neither too harsh or too lenient. But there are ways around the system that people abuse. Such as, if you Alt+F4 or Dashboard Quit out of the game, it does not count towards your 3 games before punishment. Likewise, if you close out of the game in the pre-lobby, for example if you can see you are playing against good players, it doesn't count either - you can queue dodge as much as you like. Only if you quit via the start menu and bypass the pop-up message explaining that you will be punished, will you then be punished. The only pro to this right now is that people can set up their own custom lobbies...so once Custom Games is introduced, I think the above needs to be punishable asap.