I think the notion that the Rangers don't have a potential #1 D-man and that they need to make some substantial move to acquire one is putting the cart before the horse. Having Karlsson sign next summer would be all well and good, but he's basically a decade older than the crop of forward prospects that the team is hinging their future on (Chytil, Andersson, Kravtsov, etc.), and he would take up roughly 15% of the cap in UFA. It is going to take roughly half-a-decade for the aforementioned forward group to reach their full potential, and in that time I think there is a good chance that at least one among the current pipeline can develop/project to be a top-pairing defenseman by that point.
We all want the rebuild to be expedited by nature of instant gratification, but staying the course of some extended timeline gives the team a good chance of finding the defenseman every team wants, so long as the list of potential options remains lengthy. There are ten young defensemen in the system, IMO, who have varying potential to become top-four defensemen (Skjei, Rykov, Lundkvist, Pionk, DeAngelo, Hajek, Miller, Lundkvist, Keane, and Ragnarsson) and only one or two of them need to maintain an upward trajectory into the illusive top-pairing position. I think that there is a very good chance that this coming season will shift the narrative of the Rangers' defensive woes as certain prospects rise above their preconceived trajectory and project to be top-pairing players. If I had to guess, those players will be Rykov and Lundkvist, though the other eight have as good of a chance to project as top-four staples as the season wears on.
Aside from the future on defense - what if the Rangers sign Karlsson and then none of Chytil, Andersson, or Kravtsov become top-line impact players? The team will be married to Karlsson's contract as he ages while the team frantically tries to supplement the remainder of his prime with further efforts to sign big-name UFAs, and we'll find ourselves in a rebuild amidst mediocrity all-over again.
Expediting the rebuild only makes sense if we otherwise know what we have on our hands in terms of the team's composition. We don't, though, so it doesn't make sense.