Polak has absolutely no relevance to this discussion, you're not just comparing different players but entirely different positions. Plekanec was a strong defensive 3C when he was traded, and centers have more value than wingers, so I don't know how you think that helps your case. He was certainly not a scrub player.
And for some names, past trades have Johansson and Hudler at the high end and Parenteau and Pirri on the low end. In the middle, you have guys like Purcell (3rd rounder in 2016) and Vanek (3rd rounder in 2017). To me, Vanek is the best comparable out there for Kovalchuk and he was traded for a 3rd rounder while he had 38 points in 48 games for the Wings. He was also traded for borderline nothing the next year, when he had 41 points in 61 games and was traded for Motte and Jokinen.