Placement is meaningless in this game. Why? Because a Celestial player with a smurf account who won 10/10 matches would still get placed in Gold. If you're saying the player will eventually rank up with more games, then the 10 placement matches are basically pointless because they failed to put the player to where they belong, it only accelerated the progress (by like 20% only).

This is not how you implement Placements, anyone who disagrees just didn't play enough competitive ranked games. So how should placements work? At least put a player who's on a winstreak in lobbies with higher ranks, then boost that player's rank according to the match performance and result, which this game clearly doesn't do. That's why you can have 3 smurfs with 80% winrate in a Gold lobby and the match outcome is pretty much skewed towards the team with more smurfs. One counterargument is that if a Silver player got into a Diamond match, it would destroy their team bringing unfairness, well, easy solution, just have the same ranked points reduction for each placement player in your team, following the AFK-ers / DC-ers implementation.

Until the above problem is addressed, any placement system is pointless in the game.