Once I saw how low activity was in light party after the pre-season I pretty much knew LP was going to be a joke. It was either abuse the system by swapping teammates all the time or actively win trading, which often went unpunished.

Solo works in preventing win trading in that with as active as it is, it's harder to get on opposing teams in order to abuse it. With LP this was never possible because it was pretty much always dead.

That isn't to say solo doesn't have it's problems. I know of a couple people that got other people to play on their accounts in order to get themselves in a safe spot. again unpunished but we all know SE doesn't have the drive to ban actually cheating than banning people who trash talk.