I don’t think this is the biggest issue. I don’t believe there is anything “unfair” about having a backfill swinging the match to his team’s favor. After all, the alternative is that the team with the leaver having a numbers disadvantage and it’s mostly a guaranteed win for the other team. If we are comparing how “unfair” it is, I would say that keeping the match 6v5 is more unfair than having a backfill that may or may not swing the fight. Just as it is probable that he can carry, it is also more probable that he won’t, that is if matchmaking is doing its job correctly.
What I see as an ACTUAL issue with this solution, is that it is unfair for the player backfilling. If you are queuing for a comp match, you are expecting to start a new match and see it to the end fair and square. I don’t want to be placed in a match where I’m forced to pick someone’s slack, and whatever progress the enemy team gained while that player was out of the match. Also, my ult economy would be way behind the rest of the lobby. Nevermind the fact that I could be backfilling near the end of the match where I can’t really do anything. And all of those possibilities while my SR is on the line.