This is a good question. But I did find some data to think about.
Mitchell Marner has played all 3 years with either Matthews or Tavares.
Marner With Matthews (207 games)
- GF/60 = 3.55
Matthews without Marner (222 games)
- GF/60 = 3.97
Marner without Matthews (230 games)
- GF/60 = 2.85
So Matthews has scored more without Marner, then playing mostly with Nylander. Marner scores a lot less without Matthews. This data was only 5-on-5, because they always play together on power-play, so I excluded it totally.
They 5-on-5 drop without Matthews is ~20% for Marner's stats.
So there's less scoring, but when I look on corsi, Marner makes it always better in transition, and Matthews is worse in transition without him. This is tough to count (the effect on points), but there's also more in hockey than just raw production.
But, in a nutshell, without a Matthews level scorer on his line, 100-points/80 games Marner (his last 3-season average production) could probably became 80-points / 80 games player at somewhere else.
This has happened a bit with Mikko Rantanen, now when he is not with MacKinnon and especially with Makar.
But what if Marner plays with DeBrincat, does he become better scorer? Or Larkin, could he score more with Raymond? Or having two great playmakers? What does that make on your second line?