While I can't agree wholeheartedly with DWC about our D being set (more on that, later) I can't disagree with Lagoon about using a 1st round pick on picking up a defenseman.
Back to the first point, you are penciling in JBD, Thomson and Zub. One of them hasn't played a game in the pros and the other two have never played on NHL-sized ice. They will all need time adjusting, and JBD and Thomson might need 2 years or more before they can play as many minutes as Jaros can, now. I'm not saying Jaros is better, but he's certainly ahead.
If there is no playoffs and the draft order is determined by points/game the Islanders would be tied for 13th worst with Calgary, but they have 2 more ROWs. But I doubt they go strictly on winning percentage because the schedule is heavily weighted by intra-divisional games. So taking the usual playoff format (top three in each Division and next year best in each Conference) but bagging it on winning percentage, the Islanders would be in the second wildcard spot (Carolina just ahead of them). Over in the West it would be Nashville and Vancouver, both with a lower winning percentage. So that would place the Islanders as 14th best, so 17th overall. A d-man taken outside of the top 15 will probably take 2 years, minimum, to be NHL ready (yeah, I know where Karlsson was picked, different times).