They actually did have a guarantee of a Cup in 2017 when they traded the pick for Ryan Reaves because they were picking 31st because they'd won the Stanley Cup the season before.
The Penguins were in a completely different spot from the Devils. The Penguins are on the way down. Their Hall of Fame stars are over 30 and both are signed to long-term contracts. Their future doesn't matter a great deal because it's not going to be good. Moreover, they probably had no real ability to sign an offer sheet for anyone post Phil Kessel trade because they didn't have the cap space.
The Devils aren't in this position - they are still on the way up. Their stars are not signed yet to long-term contracts. Signing an offer sheet may well increase the demands of the players they already have.
Their HOF stars are in their 30's now. Which is probably why they kept their 1st round pick for the first time in years. They weren't while the Pens were trading 1st round picks when Geno, Sid, and Letang were in their mid to early 20's.
I agree that the Pens may be on the way down. If the Devils currently were I'd say an offer sheet makes absolutely zero sense.
Pens had a team they felt could win for years to come. Which is where the Devils are. And where most teams that are continually trading 1st round picks are (the Rangers for 4 straight years while they were competitive a few years ago, for example). The Devils and Penguins aren't currently in the same spot, no. But that has no bearing on the point.
And as far as the money element of it, I've continually said that while I still would do it, there's definitely more to debate and think about there, than the picks. Though guys like Nico are in a totally separate bracket than Marner.
My contention here has always been the picks are well worth the value of the player, and to me it's not a question.