This exactly. You sign a 6 year deal in the well established price range and start playing hockey. You prove you a top pairing RHD and Chevy gets a trade offer he is willing to accept. Worst case scenario you are a UFA at age 28 in the heart of your prime and you call all your own shots for a big ticket deal.
What evidence does Trouba have that he's not going to end up stuck in Winnipeg for another 6 years if he signs that deal? Chevy isn't exactly well known for completing a lot of trades. Not to mention he has failed to trade him right now, you can't trust that he'll trade him later either.
Oh yeah, and Trouba loses any control over where he goes after he signs a 6 year deal as well. Which probably isn't very appealing.