From the Blues Perspective, The Blues love this guy as a player and as a Blue. His contract is something they could keep. He is not an FA until 2025 so there would be an opportunity to move him now, this summer, next year at the deadline or any time in between. The ticket is high (2 1st round pics) because they don't need to make a move this year. It's sorta like - yeah everything has a price. Make me an offer.
The Blues also have the cap space this summer to sign him to a longer team deal. The only question on this is his age (30) vs. the next wave of players that the Blues are building their team around.
A legit big defensive prospect and a number one pick could get this done.