To throw my two cents into this marner vs matthews thing. I think you can make a case for both, the big goal scoring center, or the crafty playmaking winger
I really don't think either of them carries more weight on that line than the other. Marner holds the puck and dishes, but often times matthews turns goals out of nothing shots.
If I had to take one when they are on their "high", I'm honestly stuck. Matthews's hart year is a no brainer, marner hasn't ever come close to that level of a player, but neither has matthews himself apart from that year. He was a true dual way threat akin to Draisaitl. If being "on" means that year than I pick Matthews as the more dominant and crucial player, otherwise I find myself leaning towards marner slightly. He can be a lesser dual threat, but my issue is 5, 10, 15 games in and he'll revert back to a pass first player, but I still think he can put together a bit more impactful play.
I think both players "lows" are just as bad. Matthews looks invisible and disinterested in his lows, Marner is actively costing us when he's on his lows. Maybe it's beacuse he holds the puck more often but when he isn't on, he's turning pucks over, making bad passes, and passing up wide shots. Matthews is just looking like he is scared to touch the puck and wants someone else to do everything for him.
And finally in terms of average player, I think this one is a toss up, but I think I'll take a B game from matthews over a B game from marner, at least from this year. In years past even if marner wasn't playing great, he was still contributing to the team's scoring and pk. This year, or at least for the majority of the year, marner has had to have a great game to actively contribute to the team's success and the pk ain't anything spectacular.
Lastly, I don't think Marner has gone off without Matthews this year, but the same cannot be said the other way around. Matthews and Nylander were popping off, but marner and tavares were struggling a bit.
All in all, I think matthews is a bit more important, but I see and agree with points on marner being more important