Marner plays like a Jarnkrok player level in the playoffs, and Celebrini is absolutely not more proven than Slaf.
Marner is still going to produce assists playing the huge minute he does with the great offensive players he plays with. It hasn't helped his goalscoring though. He has 6 goals in his last 43 playoff games. Those are brutal numbers. Nylander scored half of that amount in just 4 games in these series.
Add to it the fact that Marner is soft, doesn't take hits to make plays, loses many puck battles, and hasn't been good defensively...
Even if Slaf were to not produce as many points as Marner, his impact on the ice would be far greater because he's physical, is a beast along the boards, wins a ton of puck battles, block shots, etc. His positive impact goes far beyond points, and its not like we're talking about an unskilled player. We're talking about a 1st overall pick who scored 42 pts in his last 51 nhl games in his 19 year old season.