Fowler worked out, yes. To the surprise of basically everyone! Broken clocks / blind squirrels / etc.
I hated Gunnarsson pretty much from the moment he got here though, so I don't find his inclusion persuasive.
J-Bo had a 574-game Iron Man streak when he got here. He was widely understood to be a premier defensive player, and was a no-brainer to play in our top-4. I disagreed at the time and still now in retrospect with our usage of him, but he was a good player. He didn't take any prospects' job, since we didn't have any equivalent players in the system.
Scandella was brought in to BUF to stabilize their d-corps and failed so hard he almost dropped out of the league entirely. BUF paid a hefty price to get him from MIN, and then three years later got just a 4th rounder from MTL for his services. Then we inexplicably gave MTL that 4th back, and added a 2nd rounder for his services. There were many warning signs on his way in, and giving him a 4-year contract based on a 20-game sample size (where he scored all of 1 point) was a bad decision.
Similarly, NYI gave up Leddy for Richard Panik, who only had 4 more NHL games left in him, and a 2nd. A nominal fee at best, they were trying to get rid of him. We paid way more for him less than 2 years later, when the signs of wear and tear were already very apparent. We included Walman in that trade, who later went on to fetch SJS a 1st rounder.
I don't think it's crazy to say based on the body of evidence that we overvalue veteran dmen on the decline, and undervalue our own defensive prospects.