The defense has a ton of excellent prospects. I'd put Merrill, Gelinas, Severson, Scarlett, and Urbom up against any team's if we're talking about young defensemen not playing in the NHL. Then you've got Larsson on top of that as a player younger than most of those guys and already in the NHL.
You can buy goal scoring when you have a cheap defense. It's not pretty, but it can be done. And I don't care about point production from defensemen, not when Kovalchuk is going to be playing the point on the PP for the next 5 years at least.
The Devils look to be close to set at center and defense in the coming years. Those tend to be the most difficult things to get - wings can be bought, but it's hard to find centers and defense. Goalies used to be easy to get, now they're harder, but there's no reason to go with a guy who sucks year after year besides stubbornness. Regardless, NJ should have a top defensive team for years to come. Just like always.
Merrill and Severson are the only ones who you can count on being the bedrock of any defense corps. But it'll probably be at least another year before Severson makes the team. Plus it takes at least a year of NHL play for even the best defensemen to become dependable.
And, so long as he's not a total liability defensively (like MAB or Kurtis Foster), a defensemen that put up points can improve a team significantly (why do you think Karlsson won the Norris last year). This is especially the case when you can only count on 3 or 4 forwards to get you more than 40 points. It's not the end all be all, but it's not something where you can say "no big deal".
You're right though that the defense corps should not be a problem going forward. But, for all the hand wringing, the defense was pretty good this year, by whatever measurement you want to use, shots on goals or goals against, and especially when you consider the system focuses on forechecking and not the trap. Nevertheless, the end result was bad, and you can't chalk that up entirely to the goalies, especially if you don't think that there's anyone stellar that's available.
As I said though, scoring is what needs to be addressed. They'll certainly have enough NHL caliber centers, but, as it stands now, no centers that can anchor an offense. Relying on plug and play wingers is unsustainable in the long run. We saw it this year when the only ones we got were Sullivan and D'Agostini. If you're counting on guys like that to take you to the next level, things won't end so well.
Go all in for one of MacKinnon, Drouin or Barkov. Honestly, trade the entire draft if you have to.