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.