How well does he skate? For any defenseman coming to the Blues that may be the first question.
He's a puck-mover, also like Bouwmeester.
I really think a Russell for Hainsey deal makes the most sense IF the Blues are looking at Hainsey AND IF the Jets know Hainsey won't be re-signing.
The Blues would have to move a roster player on defense because they're not carrying eight. Moreover, as Jets fans have pointed out, their D is banged up. The dude Peluso saved is out. Also Enstrom is out for a couple weeks and Byfuglien is day-to-day. So a roster player has to come back.
Petro, Shatty, Polak, Jackman can be safely ruled out. Cole has more trade value than Hainsey. Jets aren't trading a rental for a rental (Redden). So it would have to be Russell. Russell is RFA and cheaper than Hainsey. They're both PMDs. Neither very physical. The Jets could aim for a 2d round pick from someone but they'd hope that prospect would develop into a serviceable roster player ala Russell. I could see the Blues adding a middling pick (a 3d, a 4th?) to sweeten such a deal.
For the record, I don't think Hainsey solves the problem on D but he'd be an upgrade to Russell the way each is playing this year. Prorated, acquiring him with 20 games in the season would be ~750K in real dollars but Russell's salary for the same time would be ~325K so it's the kind of affordable add they can make.
Such a deal would set the Blues up to keep using Cole, giving him the opportunity to establish as a top 4 caliber dman, and if and when the right deal came along (someone like Giordano, should he become available) you can include Cole + valuable prospect/pick for that top pairing D and it's an attractive idea for a rebuilding team. There is even the possibility Cole solves the problem internally.