If Ullmark extends, he will get at minimum 5 years, and likely a NMC.
Sogaard only has a few more years of team control. If he steals the starting job here, or at least competes for it and is blocked by Ullmark, he will be priced out here cap wise and may not even want to sign here long term.
If he doesn't get to the point where he is competing for the starting job, both sides likely move on mutually, the Senators because they aren't wasting the one spot to develop a goalie behind Ullmark on someone with no future here, and Sogaard because he will be approaching free agency and a change of scenery would make more sense than a team he hasn't made it on.
It's more of a prediction than a suggestion. I understand why people would value Sogaard more than Kaliyev.