Kostin would be a value add player to most teams at his age and contract amount. Per usual, you're taking the all or nothing contrarian stance of he's either the best or he's the worst and shouldn't even have an NHL contract (though you defend Kunin because of his 20 goal pace a few seasons ago).
Kostin was not good and was misused in Detroit. Came to SJ and put up the same points per 60 that he did in Edmonton the year prior at 2.2. His ES points per 60 is on par with Toffoli, Granlund, Hertl, etc. Not saying that he's as good as those guys as a full package, but the "waive/terminate" and "no business playing with Smith" conjecture is statistically false.
He's a big power forward that has some skill (not the most but not the least) and skating. Would be a nice F1 for a line of young forwards that are not the biggest. It's a guy that you plug in as the 3rd forward almost anywhere in the top 9 where you need to add some size and forechecking to a skilled line that might not be the biggest.
Meanwhile you pedal trading 1st rounders for Puljuarvi, Lundkvist, and Holloway because of draft pedigree without any production.
You're talking about a guy who was dumped for a 7th round pick (with the Wings forced to eat the rest of Simek's salary) a few months ago. He's not getting claimed on waivers at that cap hit.
Bizarre cherrypicking of his ES scoring numbers. Since the start of the 21-22 season, which encompasses 149 of his 155 career NHL games, Kostin has scored 1.65 points per 60. Which happens to be identical to Mathieu Joseph who the Sens just had to attach a 3rd to dump on the Blues.
Kostin is the least useful forward among our one-way contracts (Couture, Toffoli, Granlund, Wennberg, Goodrow, Kunin, Sturm, Grundstrom, Zetterlund, Dellandrea, G. Smith) because he doesn't have a track record of filling a defined role in the NHL. That said, this is a massive credit to Grier considering Kostin is still a huge upgrade over some of the garbage we were trotting out up front last season like Hoffman, Barabanov, Labanc and Zadina.