Blues have no reason to give up Kostin, and a First for a rental contract, and a spare part.

Feel like the blues could resign him and fast is a hard nosed bottom 6 player that’s plays the pk well

Yeah, but we have a great PK, and a Great Bottom line. and we just gave tryouts to Brouwer and McGinn hopefully one of those two can solidify that bottom line and when Steen comes back it would be all good. And the point of bringing in Kreider is pretty much nullified once Tarasenko gets back especially when he is a Center we have O'Reilly Thomas (who should be playing 2C here soon) Schenn Bozak Barbashev Sundqvist has played Center with Kostin coming up who also Played quite a bit of Center alongside Alexandrov who is rumored to be potential of 2C Joshua who is a MIddle 6 Grinding Center Toropchenko who could be middle 6 as well. I think he would just be a redundant player. It only makes sense for the Rangers doesn't really make much sense for the Blues.

to be fair - Fast is hardly a spare part...

to the Blues it's a spare part he wouldn't even make it on our third line.