I think the Blues will give Kostin one more season. It's weird with how little of a leash they gave to Thompson and Bokk how long they have given Kostin tho.
Tho, I don't really think it was anything Thompson did, and more that RoR was worth losing Thompson.
I don't think that trading Thompson/Bokk was an indication that the organization gave them less leash than they have/do Kostin. I think that those two were more sought after than Kostin by the teams that acquired them.
All 3 of those guys never rose higher than 2nd tier prospects. Each was drafted in the late 1st round. Thomas/Kyrou both had fantastic post-draft seasons to elevate their value/reputation league wide and make teams regret passing on them. They became 1st tier prospects. None of Thompson/Bokk/Kostin did anything in the couple years following their draft to increase the value/reputation they had on draft day, so they remained in that 2nd tier of blues prospects. All 2nd tier prospects are available in the right deal and (for the most part) it is the team trading the NHL player who picks the 2nd tier prospect they want. Obviously there are exceptions, but it is much more often "pick one of these X prospects" than it is "we'll give you this prospect and this prospect only." We know that Kyrou/Thomas were the only "off limits" prospect for ROR and we haven't ever heard that Kostin was off limits for Faulk.
If Buffalo wanted Kostin more than Thompson, that isn't a deal breaker for Army. If Carolina wanted Kostin more than Bokk, I doubt that is a deal breaker for Army. If a team trading a top 4 LHD in the last year wanted Kostin, the Blues absolutely would have been willing to make him the prospect in that deal.
There is no doubt in my mind that Kostin would have been out the door if he was the prospect targeted in a trade for a #1C or #2D. I don't think any team has presented us with an offer where we acquire an impact NHL player in a trade where Kostin is the centerpiece.