If someone mains mid where they gain or lose 18 lp per game, they play 10 games in a row as mid, and now are autofilled to support, where even if they win, it's only +3 lp to their mid lane score.
If the auto filled support isn't winning by 20 minutes and his team just made a hard throw, he will be FAR less likely to try his hardest to pull for a 45 minute win, rather than try to end the game as quickly as possible, by a few 'oops lag sry guys' mistakes to get back to games that 'actually matter'. Oh, of course, you'll have the more vocal trolls who will make this intention clear, but more often you'll just have people soft-trolling and just happening to be 1v1'ing an enemy when baron is up and all are mia for no apparent reason, that kind of thing.
Game quality is going to take a massive drop if there are any autofilled players in all elo's but the highest 0.1% because people just simply won't give a shit if the game takes longer, since these games literally don't matter to them. I expect afk's and trolls to massively increase after this 'improvement' is implemented.