Solution to people leaving Team Builder lobbies
Around 90% of players queueing up for team builder will leave in <5 seconds if the lobby isn't full. The reason Riot gives for allowing people to leave lobbies without consequence is that players should be able to choose to play with a team comp that they are happy with. This is great and all, but almost every time someone leaves, it isn't because they aren't happy with the team comp they see, but rather it's just because they are impatient.
In my experience, if I'm with one friend in a team builder lobby, and need 3 other players, we have to wait 15-30 minutes to get a full team every time. The thing is, if people didn't leave instantly when they see that the lobby isn't full, the wait time would probably be closer to 5 minutes.
All that needs to be done is allow players to leave with simple increasing wait penalties that start really low. Based on my solo team builder experience, it would be rare to get 2 or more lobbies in a row with undesirable team comps, so a system such as this should be fine (this may even be a bit lax): -First leave is free -Second leave has 30 second penalty -Third leave has 1 minute penalty -Fourth leave+ 5 min penalty (meaning people will just stay at this point, but if you got unlucky and left 4 times, its not that bad of a wait.)