Lets leave aside the McDonagh argument because I just don't agree, but what particularly am I sleeping on about Howden and Hajek? Are they sure thing impact guys? Howden is a sure-thing top 6 forward? Hajek is a sure-thing top 4 D? Thats before we even get into a discussion about whether they can be 1st liners/top pair d. I think the answer is no.
With Sergachyov, you are getting a 1st pair defenseman. The kid is 19 and put up 40 points playing 15 minutes per game. Some of that is playing for the Bolts, but part of it is that he has a good offensive skill-set and is already showing it in the NHL. He also is big, physical and good defensively. He wasn't required to play big minutes for Tampa's D this season due to their depth, but he hasn't been a defensive liability this season, and for a 19 year old D in the NHL thats very impressive.
Why in the f*** would the Lightning deal a 19 year old 1st pairing guy that put up 40 points for an often injured 29 year old first pairing guy, who has been regressing? This is madness.