I think they probably find ways to get more 1st round picks, if they give up all those guys. In hockey, 3rd and 4th round picks have like, a third and a quarter of a chance of becoming NHL regular players, or maybe worse than that.
I don't think there's anybody on this roster McPhee wouldn't trade for a 1st round pick and an average prospect. Trading Marchessault isn't crazy at all, since he's an unrestricted free agent at the end of the year, who took until he was 26 to make it to the NHL full time and then put up 30 goals in a season. Who can really tell what his next contract will be worth? I'm guessing the low end is about 3M, and at the top, maybe 8m? That's a real headache for a GM, and while that goals column is fantastic, Marchessault is still a novice, defensively, starting 58% of his shifts in the offensive zone, while his team only barely leads in shots, with 52%.
https://www.hockey-reference.com/players/a/audymjo01.html
Most NHL players play their best at age 27, and that's how old Marchessault is right now, so his value is probably at its highest at this spring's trade deadline. I predict that he goes for a 1st pick and an AHL player, unless he's scored another 30 goals, in which case you can add a later pick and/or an NHL player to that recipe.
The Knights want to compete and see what they have this year, but unless they look like they can win a series or two in the playoffs, trading away guys who are going to ask for big contracts next summer and might not be around long enough to work with the three first round picks Vegas took in the 2017 draft when they're ready to really make some noise... that probably makes sense. It's not just about getting all the good players you can. Winning the Cup seems to take a lot of good young players at once, so I expect the Knights to try to take three or even four guys in the first round next summer. If they have a good year together, they can rent out the expiring contracts of Marchessault, Neal, and Perron for first round picks. Garrison, Stoner and Sbisa could bring back later picks and/or prospects, and could be replaced with other free agents next summer.
Six first round picks over two years was the recipe McPhee used for the Capitals, and it worked pretty well, so I could easily see him trading some very good players to try to replicate his past success. Now, trading Marchessault for anything less than a 1st pick... that's just crazy talk.