He is a good all around D. And he is younger than Terry. And he plays a position that is much harder to fill than a winger.
I don’t think a 1-1 Terry for Dobson deal is enough for the islanders but that is the cost of acquiring top talent. Terry’s replacement is already in the system. Guys like Dobson are almost never available.
The "replacement in the system" argument is a risky one. Sure, Sennecke looks good and maybe he's amazing for the team. Or, he gets injured and never lives up to potential. Or, he demands a trade two years from now. The problem with this viewpoint is that you hope the player stays and becomes good. Terry is already good. There's way less risk.
The Ducks defensive options are plenty between Trouba, Gudas, Mintyukov, LaCombe, Zellweger, Helleson, and Solberg and then the other guys already drafted. If they want an upgrade, wouldn't this be the area to trade from? Otherwise, it becomes overcrowded.
At the same time, there is a clear need for more top 6 forwards. Trading from this already thin area seems like a step backwards. Additionally, top 4 defenseman are traded all the time. The Ducks got Trouba for very little. However, scoring forwards, they go for high prices and are not often traded. If it was easy, why has Verbeek been unable to do this for over a year?
Guys like Dobson are available regularly. Guys like Terry are not because teams fight to keep them.