Old adage of drafting the best guy and sorting it out later. Nashville in 2013 was probably aiming for a forward, but Seth Jones was unexpectedly there at #4. Less than three years later, Jones was traded for Ryan Johansen. Probably a better end result than them taking Elias Lindholm over Jones.
Some of the worst picks were a result of teams pigeonholing themselves into a certain position. LA badly wanted defense in 2007 and bypassed somebody like Voracek for Thomas Hickey. At the time, LA had plenty of young forwards. Similarly, I remember arguing with an Ottawa fan around 2006 about them bypassing Kopitar for Brian Lee. The Ottawa fan scoffed saying that led the league in goals and had more need for a defenseman. By 2008-09, Ottawa's offense fell off a cliff.
Whoever we're taking (forward or D, at #11-12) is probably a few years away from really being a difference maker. We have no idea what our team might look like after trades/free agency.
Andy Greene is going to be 34 at the start of next season. By the time Chychrun (or whichever D) turns 20, Greene will be 36. The thought of a Chychrun-Larsson long term pairing doesn't sound awful.