My goodness people are overly defensive.
If you care to listen to my reasoning, and not just immediately dismiss people who don't 100% agree with you, then read on.
Marner struggled to score last year, and he was forcing a lot of passes. That doesn't mean he was incapable of putting up points, or being effective, because he is still one of the best playmakers out there... But Tavares was absolutely not the one who was running cold on that line.
As for not being able to carry a line, I think he needs a really good finisher to reach anywhere close to his peak performance. I think if you don't give him that guy, he is nowhere near as effective as what he is right now playing with Matthews, or playing with Tavares last year. For example, if you gave Matthews a couple of line mates like Kerfoot and Hyman, I think he would still find a way to rack up points. I think Tavares would be capable of doing that as well (when he is not on a cold streak obviously). I am not as confident in saying that Marner, or Nylander, could do that. That doesn't mean they don't make Matthews and Tavares significantly better than Kerfoot, or they can't still be very effective and productive players. That is why they are worth their money (or at least the vast majority of it). But true line drivers need to be able to do everything and I don't think Marner has enough of a shot to be able to do everything. I don't think there are many wingers in general who are capable of being line drivers all on their own, and it is not a reflection of ability or skill as much as a player profile.
If you don't like my opinion, that is fine. I definitely think comparing it to "garbage" or "convincing people that the earth is flat" is wildly outlandish and extremely reckless.