The defensemen that will be available later in the first round, and even in the second round, may very well prove to be superior to Nemec and Jiricek. It is much easier to go mining for defence with later picks than to try to unearth a top talent on offense once you pass the top ten or so.
I don't entirely agree on your angle. but it's defendable if you consider Cooley/Savoie/Slafkovsky/Nazar as potential 1st liners, players that dictate the game.
Otherwise you definitely take the first pairing D ceiling prospect over a 2nd liner.
Personally I'm not ready to say those forward prospects (meeting let's say 80% of their ceiling hopefully) are shoe-ins for the 1st line, Slaf being the most likely imo.
Your statement is sometimes true but shouldn't be considered gospel truth. Look at the Sergachev draft (2016) - the only first pairing D after pick 16 (Chychrun) is Adam Fox with a whole bunch of misses in between. 2017 is even worse with *maybe* David Farrance past Heiskanen/Makar. 2018 doesn't look brighter. Even the incredible 2015 draft doesn't have a top 2 defender past Chabot at 18. For top defenders you also have to pick early now - the Shea Weber and PK Subban picks are an outdated reference.
By our 2nd round pick the only Ds with legitimate 1st pair potential still available might be limited to Hutson (smol boi), Bichsel (mostly defensive but yuge), and Lindgren (big if). Nemec/Jiricek/Mintyukov/Mateychuk/Korchinski/Pickering I suspect will be gone and I wouldn't trade up for the last 2. Definitely more than 6 Ds drafted in the first 32. Absolutely no point for the Habs in drafting a defender with a 2nd pair ceiling, at this point. Go big or go home. If that's what you see in Nemec and Jiricek, I understand your position.
In my opinion this draft will have sufficient offensive quality at around pick 32 that I would prefer:
Nemec/Jiricek + Goyette/Ostlund/Kulich/Firkus/Gleb and other falling Russians
vs
Cooley/Nazar + Hutson/Bichsel/Lindgren
Slaf is the wildcard, I don't know yet.
Anyways, BPA.
And let's suck again next year.