I've posted these stats many times but maybe you haven't seen them:
5v5 over the last 3 seasons to get the sample acceptable:
Player 1 | Player 2 | Player 3 | Player 4 | GP | TOI | GF | GA | +/- | GF/60 | GA/60 | GF% |
Auston Matthews | Mitchell Marner | w/o William Nylander | w/o John Tavares | 205 | 1808 | 120 | 74 | 46 | 3.98 | 2.46 | 61.86 |
w/o Auston Matthews | Mitchell Marner | w/o William Nylander | John Tavares | 215 | 889 | 45 | 29 | 16 | 3.04 | 1.96 | 60.81 |
Auston Matthews | w/o Mitchell Marner | William Nylander | w/o John Tavares | 227 | 829 | 50 | 33 | 17 | 3.62 | 2.39 | 60.24 |
w/o Auston Matthews | w/o Mitchell Marner | William Nylander | John Tavares | 237 | 1559 | 72 | 83 | -11 | 2.77 | 3.19 | 46.45 |
Nylander plays with Matthews about as much as Marner plays with Tavares.
Matthews does well with everyone but a bit better offense with Marner.
Marner with Tavares is as good as Nylander is with Matthews. Think about that.
Nylander is horrible with Tavares.
To your point. No one thinks that Matthews is overrated. And you've now been presented with the facts that say that Marner isn't overrated.