Draisaitl, Matthews, Barkov, McDavid, MacKinnon, Eichel are the only forwards better than Marner IMO.
Now, can Marner generate 100+ point seasons without Matthews? That remains the question.
Whenever Matthews is injured, you'll see Marner take more initiative to score, so yes, I think he can generate similar offense with subpar players.
Now he is not a selke level defender, but, make no mistake -- Marner is extremely competent in his own end. He doesn't make any mistakes, but, once again, he isn't a shutdown guy.
He is just sound defensively, meaning he is overall a net positive defensively. Not a lot forwards including Kucherov, Kaprizov, Draisaitl are as good as Marner defensively.
Now take the example on the Leafs of Nylander v Marner, Nylander is a better skater and shooter, and he's also a great playmaker.
But, what seperates Marner from Nylander, is his defensive and offensive awareness.
He will never make any mistakes defensively or offensively.
However, at times, Marner's compete level is very questionable, he lacks the desire to win and the motor of a franchise player i.e. Mcdavid/Draisaitl/Matthews etc
So all in all, I don't think to a contender he is worth 14 million, it's better spent in other facets.