plus 4 first rounders each of which can be as good as Mitch Marner
This is just irresponsible commentary. Any team looking to add Mitch, for the sake of argument, will draft 20+ all four years. For some perspective, lets look at the 12-17 drafts, 18 and 19 are both too young of draft classes to really judge, but I think this should be paint a good picture.
From 20-30/31 overall, the following players of note were drafted.
Maatta/Skjei/Pearson (10 picks)
Mantha/Burakovsky/Theodore/Hartman (10 picks)
Schmaltz/Fabbri/Kapanen/Pastrnak (10 picks)
Boeser/Konecny (1o picks)
Jones/Steel/Borgstrom (10 picks)
Thomas/Poehling/Frost/Tolvanen (11 picks)
So in 61 picks over those SIX drafts, there were only 20 'notable' picks. Some journeymen, some kids who could end up being decent players (Juulsen as an example for Montreal) but those are the NHL regulars, NHL 'stars', or players who could very likely turn out to be major plays (Top6/Top4)
So with that having been said, there is an approximately 30% chance that a team picking 20+ across those six years would have selected one of those players; the other 70% of the time you're picking someone not worth this list.
Across four years, with perfect luck, you're getting FOUR of those players.
Across four years, with terrible luck, you're getting ZERO of those players
Across four years, with AVERAGE luck, you are getting 1.3 of those players.
Let's say you could have any TWO of those players --- if you remove Pastrnak who is the outlier, I don't think any package of those two players get you Mitch Marner.
I would argue that for MOST of those players, I wouldn't even take a package of THREE of them for Marner. (Would you take Kapanen, Fabbri and Pearson or Maatta for Marner? Nope)
Sure, you could get the next Pastrnak --- hell, maybe you can get four of them with your first round picks... but it's not going to happen.
The dangers from trading first round picks is that if you aren't attempting to add to your cupboards, its very likely you run them dry (see: Toronto currently) but for a team like say Montreal who has tons of prospect depth, giving up four first round picks for Marner is a good deal. A fantastic deal, even.