Where does this idea come from that the Caps are in the market for a top-pairing blueliner, anyway? There's no indication this is the case. They really like Fehervary, who is young and already a solid partner for Carlson. He's as unlikely to be traded as any player, I'd suspect. Young, cheap, bright future.
That's before getting to McMichael, who has a long future as the team's No. 3 or No. 2 center ahead of him. There was some thought that he might be available to fix their immediate center dilemma if they went shopping for JT Miller, but the price kept rising (or never came down) and they pivoted to Strome.
On defense, they likely have a need for a depth guy, maybe a No. 5-6, possibly a No. 4, though they would need to trade someone to do it. I'd think another veteran in free agency making <$2M is more their speed.
The Caps' days of spending all their futures for immediate help are past. They're balancing between competing and gathering players for the future now. Adding three years of Chychrun just means he leaves at the same time as Ovechkin and Backstrom and Oshie and the rest of the legacy core.