This is exactly like a Pens fan saying Naslund was a good deal because we got something out of nothing, that Naslund would never develop into something good etc...
There are so many examples of players turning their career around (Kovalev, Straka, Naslund, St Louis, Tarsntrom, etc...) that it takes one hefty does of blindness and/or bitterness to say a guy won't amount to anything when he is that talented and had such a stretch last year.
Jackman was the main reason for the Pens' turnaround, even though he wasn't the only one.
He played like a top defenseman, and actually played better and better once his confidence settled firmly.
Pittsburgh is the perfect team for a player to turn his career around, and there are many examples. Straka explained it perfectly when he said the management was around the players and that there was no pressure compared to some other places. Players are able to express themselves freely both on the ice and off it.
Skilled players like Lang, Straka, Kovalev, Tarsntrom or Jackman are able to try offensive moves they would not try in another situation because offensive talent is sacred in Pittsburgh, even if it means making some mistakes.
Jackman did just that, and the mistakes became fewer and fewer. Simply put, he played last year in Pittsburgh the way a guy with his draft status should be playing.