Except for the fact that in real life, we aren't flipping Karlsson for more than we paid for him. As you've noted, the assets going to Ottawa aren't close to enough. Even if we could potentially gain assets by trading for him and then flipping him, what would be the point of having him only for a few months? If we are going to trade for him (we aren't) it would be with the idea of keeping him, and that doesn't make sense for a rebuilding team. We didn't want to pay McDonagh 7+ mil into his mid 30s, but we are going to trade assets for Karlsson and pay him 10+ mil into his mid 30s? Where's the logic in that? Yes, Karlsson is a better player, but he's every bit as likely to regress as he ages. He's had some significant injuries in his career.
If we were at the end of the rebuild and needed that extra player to put us over the top, then a move like that would make sense. At the beginning of a rebuild, no.