Really? I mean you're getting Skjei as well while dumping Callahan. If anything it seems like it's rougher for the Rags as it's Skjei+Kreider for Sergachev and a cap dump.
Sergachev has a ton of value with us. More than people seem to realize. Half our fanbase didn't want to include him in a deal for Karlsson, and it appears Yzerman was on board with them. (Personally, I would have pulled that trigger, but just using the example to illustrate a point--we see him as a huge part of our future on D.)
I also don't think we're at all interested in dumping Callahan right now. That's for the off-season, when we need the money. He's actually still a contributing member of the team so long as we keep him healthy (by resting him a lot), and I don't see us paying a premium to get worse for this year's playoffs. Losing him does make us worse, and we won't need the cap space until next year.