I don't like that particular trade, but I think that we can come away a better team with the proverbial 4 quarters trade.
Following the Eichel template
Top-6 Forward making 5m for 3-5 more years + top prospect ready to step in + 1st rounder+ 2nd rounder +Use the 5.9 we save on another forward at UFA.
No one player as good as Marner, but (assuming Nylander is re-signed) we still have an elite 1st line RW, two new proven top 6 guys, Knies + another top prospect stepping into the lineup cheaply, and a 1st + 2nd that can either be used to restock the pool for a wider contention window, or provide the base of a deal for another d-man.