Both contracts are an issue.
Not really. Tavares is still generating offense at an elite rate. He had some finishing issues, but that was mainly a sh% issue. This season was Tavares' 3rd lowest of his career, and well below his career average. So unless you think Tavares has forgotten how to shoot effectively, it's likely we are going to see that sh% climb next season, and his usual goal pace return.
The issues with Marner are much more deep rooted. On the surface, Mitch Marner produces and performs like one of the best players in the league. His defensive ability is almost underrated at this point, and his offensive ability speaks to itself. He also is a respectable PKer. However, there appears to be a mental block with Mitch Marner in which the player he is in the last few playoff series, versus the player he is in the regular season, are entirely different.
The best case scenario, is Marner remaining a Leaf, and gets passed that mental block, and can be the player he is, in the playoffs. He's shown flashes before (the 17-18 series), so we know it's possible.
However, that brings us into a discussion of whether it's possible, in this flat cap world, to build a team with 3 11M players. But that discussion is more about team building strategy, and doesn't affect whether a player is worth it or not.