The problem with your argument is he's really turning into a consensus top 10 pick. Bob McKenzie's list, Craig Button's list, Corey Pronman's list all have him as a top 10 pick. Beyond the top 2 or 3, the rest of the top 10 has been considered a complete crapshoot. You've seen the varying opinions on the Red Wings based threads of people arguing between Boqvist and Bouchard and Dobson and Hughes. Wahlstrom and Tkachuk have gotten love at the forward position. With how Kotkaniemi has been viewed as raising his stock, he absolutely is in the conversation. I don't consider it a reach to take an emerging consensus top 10 pick and best center in the draft at #6 in a draft where nobody knows what to expect.
I'm not basing my decision off of a comparable, rather what he's capable of. But even if I were, how does me saying "Kotkaniemi's game reminds me Kopitar" make a player busting mean anything more than "Kotkaniemi is a strong, high IQ center that is a solid defender and excels as a playmaker," or even "Bouchard is the offensive minded defenseman that can run a powerplay that we've needed for so long"? One is a list of things a player does well, the other is a current player that embodies the things the player does well.
And no, we don't NEED a defenseman with the first pick. We NEED a player with front line talent; whether it's a top line winger, a top line center, or a top pairing defenseman, it doesn't matter. Get whatever player the scouting team has determined is the best player for the organization.