You could literally say this about anybody they pick. The simple fact is, Cooley offers us nothing we don't already have. How does he become a superstar and clear cut best player when he likely doesn't get a PP role that fits his strengths since we have two different options that are clearly better? That's the point. Unless we hit a dead end in contract talks with Jesper Bratt and there's no way he's coming back, there's no place for Cooley to become that "if" that you just described. There is a such thing as too much of the same thing.
This draft's forward crop is separated by a razor's edge from Slafkovsky to Nazar to Cooley to Savoie. When all things are equal, you absolutely take into account the needs of the team and the system. This idea that you just throw out need as if it plays no part in the evaluation process is just pure fiction and naive.
Regarding the first part, so are you saying that if Capitals were given a possibility to draft Ovechkin's literal clone, they should say "no, thanks" because they already have one Ovechkin in this spot? Nashville literally drafted Seth Jones into the logjam of D and fixed that by trading him for perceived young #1C Ryan Johansen. They had an enormous need for a center but still, they ignored their need because they believed Jones is a better prospect than Monahan or Lindholm.
Regarding the second part, this is like, all your opinion. If you believe that multiple prospects are equal, then of course, "a need" is the only thing separating them but:
1) I don't think that many people believe that Slafkovsky/Jiricek/Nemec/Cooley are equal (before even adding Savoie, Nazar to this crop). Judging by what people write here, most people prefer player X over player Y.
2) If you spend thousands or millions of US$ to run your scouting department, them coming back saying that "You know what, boss, Jiricek, Nazar, Cooley, Nemec, Slafkovsky are all entirely equal, pick up whoever fits the need." is absurd. IMO, "equal" prospects exist only in rationales of people who want to push the "team need" narrative. And that's fine, this narrative is fine for mock drafts (otherwise those would be rankings with players assigned to teams) and building a team on the forum, however in the real life, BPA works better because if you maximize value via draft, you can fix the team needs by trading value from the area of strength to the area of relative weakness.