Autofill changes player behavior in a negative way
I want to start this post by saying that I personally do not mind being autofilled. I'm ok with playing Support, or whatever other role autofill puts me in (I once got autofilled into Top lane, somehow). I'm a flexible enough player to pick a champion suited to the lane I'm assigned and play it to the best of my ability.
But not every player is, and this is becoming more apparent over the past couple of days. There's a disturbing trend I've noticed with players who get autofilled into roles they don't want: They pick troll champions designed to scare one of their teammates into leaving champion select.
Case in point, the last game I queued for, I got assigned to Bottom lane. The first pick on my team was a Support player, who I can only assume was autofilled, because they locked in Riven as their support pick. Now I have to live with the prospect of having to play an AD Carry with a Support champion that literally cannot play the role, and has no function unless they magically get fed and turn into "Top Lane Riven as Support".
So as the bottom lane player, I am presented with a choice. Either: A. I play out the game in a massively disadvantaged lane with a Support that has no tools to protect me, and waste time for a game that is almost certainly going to be a loss B. I leave the game in champion select, and eat the timeout that's assigned to me for leaving a game (And this is the 3rd such situation I've had to leave today, so now this is turning into a substantial amount of time)
Autofill is rapidly becoming a game of chicken. The Autofilled player can just pick a troll champion, hope someone disconnects, and will likely not get autofilled in the game that comes up next in their queue, allowing them to play the roles they initially wanted to play. Meanwhile, they force some other player to eat the disconnect penalty because of their troll pick.
Autofill is not a viable solution at low elo, because you can't force players to play roles they don't want to play. They will take every effort to ensure they don't ever play the role, and if they do, that everyone on their team suffers for it. It leads to horrible game experiences for everyone on the team, unless one player decides to take one for the team and not let the troll pick be played.