Not to be argumentative but the Oilers won a cup after they trading Gretzky, Chicago won a cup after they traded Buff, Anaheim won a cup after Kariya left, granted 5 years after. There is a long list of teams who star players left, only to win the cup afterwards. Losing a trade does not necessarily mean your team gets worse.
I don't think its that Matthews and Marner are the problem from a skill perspective, its the cap inflexibility that having so few people eat up so much of your cap, that is the issue, again my opinion. The gap in skill/ contribution between Matty/JT/Marner and the bottom 6 is massive. Jarnkrok, Kerfoot all being asked to play top 6 minutes, when in reality they are no where near top 6 players.
What this team needs desperately is controllable, cheap, very good to elite talent, to compliment the big ticket super stars, this is where Dubas failed, his asset management was piss poor, he thought we were close and traded away picks and prospects to supplement the elite talent and in most cases it did not work. Now our farm system is a who's who of nobody's below 5'10" .. some might become bottom 6 tweeners but outside of Knies not much to look forward to.
We need $750k young good to elite talent that plays like 2nd liners on the 3rd and 4th line, we need those players bringing a brand of aggression, talent and defensive acumen that wont be a liability, which is asking way too much of players that are inexperienced.
I have no issues trading either player. If Vaive, Sittler, Gilmour, Clark can all leave, so can Mitch and Matty.