Yeah see, alot of people think that queue times would just get longer if they removed autofill.
They'd get longer alright.
And longer.
And longer.
Until eventually it'd take a full day to match a lobby together. And it wouldn't stop there.
You see, the matchmaking queue is like a traffic jam:
When cars are arriving at a traffic jam faster than they are leaving, the line continues to grow until the obstruction is cleared and cars can begin leaving at the same or greater rate than they are arriving. The first few cars might only take a couple minutes to slowly crawl their way through, but after a few hours you're getting cars pulling into the line that will have to wait hours to pull through. And in this analogy the obstruction never gets cleared. A fairy just comes along and whisks everyone back to their homes to reattempt the journey every two weeks.
You might be willing to wait 30 minutes to get your chosen role, but would you be willing to queue up knowing it could be 30 minutes wait, OR days, OR never and you just end up waiting until patch day comes and Riot resets the server for maintenance?
I know I wouldn't.