Ryan Murray proved he could play in the NHL as a 20 year old, so did Tyler Myers, so did Damon Severson, etc.... Proving you can play early just means you have a solid floor, that doesn't mean you have the qualities to project as a future top pairing defenseman. What about Nemec's game specifically makes you think he'll be an elite defenseman?
Like I said before, trading Nemec for a forward with term isn't giving up on him, It's trading from an organizational strength to improve an organizational weakness. You talk about worrying about options on the right side but how about the fact that we have a huge dropoff in talent on the current roster after Nico, Jack, Bratt and Meier with only two decent forward prospects in Gritsyuk and Lenni (neither of which are better prospects than Casey currently is). The forward group is a much larger weakness but we're going to hoard defensive prospects because Dougie is 32 instead of fixing an actual large current issue?