We aren't trading Marner to improve on defense. Maybe we consider moving Nylander at some point but I highly doubt it. Look at how the top teams have built their defense. Also, realize we have already allocated a top 5 pick on one (Rielly) who is still developing. The only top team who has their D built around mostly high picks in Anaheim (Lindholm at 6, Fowler at 13) and St Louis (Pietrangelo, traded a 1OA in EJ for Shattenkirk). Chicago, LA, MTL, San Jose, Nashville, Tampa, and Pitt have mostly acquired their top defenders through 2nd round picks and good trades (not overpayments like Hall for Larsson). Subban, Weber, Keith, Josi, Letang and Vlasic were all 2nd rounders. LA and Tampa got franchise defenders at the #2 spot (maybe we can do the same with Liljegren) but most of that has been supplemented by good additions, not in the first round. When you actually look at the case of LA, their attempts to supplement their talent with mid-range firsts it has been largely unsuccessful (Forbert, and Teubert) instead they have found success later on with Voynov (now banned), Muzzin, and Alex Martinez. San Jose made a great trade for Burns (trading nothing crippling) and acquired Paul Martin as a UFA. Pitt has a fair amount of high picks on D, but they won with an average D who could transition the puck. The only first rounder was Juolevi. Their highest pick in recent years was commonly scratched (Pouliot). Chicago's only 1st rounder who was played consistently was Seabrook.
I also never said that any player in the 5/15 range was a sure thing. But, there are probabilities attached to each player. Without even factoring in the players in this draft, defensemen are in general of much higher risk than forwards to bust, and it is more likely to find a star defender in the later rounds than a star forward. When looking at this draft and examining the talent, its quite clear most of the top talent are forwards. While i'm fine with taking Foote in the 8-15 range, we shouldn't do it unless he is the best player on the board.
You seem locked in on the idea we have to trade something of significant value, drastically overpay in UFA, or have to draft a D in the top 15. I think, one those aren't the only option, and if they are, I would choose to overpay in cap space at the moment then take a need based approach to the draft while handicapping Hunter's skill. Personally, I think smart teams find a way to build good defenses without going to the extremes you've claimed.
