The SR adjustments could be minor for backfill players, like in QP.
I’m sorry, what? There is no such thing as SR in quickplay and the range of MMR you can match with is much wider than in competitive. I match with people 2 tiers above and below me all the time in quickplay. This almost never happens to me in competitive.
Consider this: most people leave in competitive because they are tilted or don’t care anymore. Very rarely do you see a leaver from the winning side although it does happen too. Who is going to want to backfill into a losing team? When people rage quit, they usually do so after a considerable chunk of time when they feel like there’s no chance of winning. They do this to spite their teammates who they’ve likely been quarreling with, as a last resort to get back at the team. Besides, what kind of message would blizzard be sending by implementing a backfill system? “It is OK to leave because someone will replace you.” If you’re playing competitive, I expect you to stick until the bitter end. If you’re the kind of person who gives up and tilts easily, I don’t ever want you as a teammate.