It's a step in the right direction but I dont think this action is enough and I dont think it was fast enough. These players BROKE the matchmaking system and they have been doing it for weeks. What Elo booster is going to look at the ban and be sad? Elo Boosting had a high premium, so this is just the cost of doing business (not to mention their smurf accounts can enable them to avoid a large portion of the penalty). Based on what I read in the OP, the boosters dont even lose their season end rewards?
Logging onto accounts that were clearly below their skill level made them essentially the sole determination of how a game goes. I'm not going to be outlandish and say that this kicked bystanders below where they belong, given how unlikely it is to continuously run into this situation. However, I am going to question what the point of ranked is if you dont protect players from significantly imbalanced teams?
Why not take a hard public stance early, and follow up on your stance once you had the evidence? It seems you spent the time building the case, while people continued to abuse the system and reap the benefits.