it is impossible to make a flawless system though and it does detect most smurf accounts very fast and give them enough mmr to leave the earlier elos, the problem comes when they play on other people's accounts like these people are
I personally think account sharing should just carry a suspension with it but not everyone agrees apparently so you're just gonna have to live with there being some boosting going on
the reason it may take longer for the system to move them up into the rank they should be could be due to a variety of things like the sage's mmr being too low so the game gives worse opponents, the account they are playing on being a bought account with lower mmr etc
All that said, if an account has top 1% damage/win or only mvp in carrer must be placed in the correct rank. It's so easy to understand that this conversation is useless.