I wouldn't generalize like that - I'm not saying trade out of emotion, I'm saying consider everything and don't rule out trading core pieces because you should never rule out anything.
You're again stating opinions as facts - you don't know what trades might be out there for consideration.
There are a number of scenarios for management to evaluate. One possibility is elite players who aren't happy where they are, we know of Jones and Eichel, perhaps there are others. The other end of the spectrum is to just trade say Marner for a boatload of picks and use the cap space to sign an elite player like Dougie Hamilton. Then there are all kinds of other possibilities and there might be several that make us a better tean.
If a player like Marner was made available, I can guarantee you that there would be a TON of interest, there would be many interesting offers to choose from and saying none of them would help us makes no sense whatsoever. And the same goes for Matthews to an even greater degree. When you're a killer team on paper yet you keep losing in the 1st round, doesn't it make sense to consider other options? Say we lose again in the 1st round next year to Boston, TB, Florida, WSH or whoever else, then what? Are you still gonna say can't trade core pieces? At that point Marner and Matthews will have less value as they're under contract for one year less. And if you don't change anything again and lose again, then their value goes down again and Matthews would basically be a rental player.
Every year you wait, your trading position gets worse. If you are going to stick with this core until the bitter end, you better be damn sure that it's the right thing to do. I don't see it myself and think it's time to consider all our options.