Rantanen is not worth more than Marner. Mackinnon is hard carrying him, without Mack Rantanen is scoring 75 points. I'd much rather have Marner. He would be scoring 120 points with Mack.
How about in the playoffs though? There are 4 active players in the top-10 of playoff PPG all-time. McDavid at #3, Draisaitl at #5, MacKinnon at #6 and Rantanen at #8.
Let's also not pretend that Marner is playing with some scrubs either. MacKinnon is the second best forward in the league, but Matthews isn't too bad either. If you think Marner is better (or deserves more) that's fine, we all have our opinions. But I can guarantee you that outside of Toronto, 90% of the people would pick Rantanen over Marner. Actually, probably even the majority in Toronto would do that considering how Marner is perceived there these days.
I wouldn't be surprised if Marner ends up getting more than Rantanen though. Either Toronto will extend him, and he's bound to get a raise, or someone will back the truck for him in free agency. Meanwhile the Avs are trying to squeeze Rantanen at least under MacKinnon.