Of course it's not a vacuum, I never insinuated otherwise. Its entirely possible that Drury signs before he gets to his senior year and this is all moot. he could also say "this team isn't going to trade me so I guess I'll just keep playing in college until I'm a free agent". He could also win the lottery and decide he never wants to play professional hockey, I guess. There's a multitude of scenarios that could occur. I highly doubt that Drury is going to look at this situation and say "man, what a bad organization, I'm not going to sign here." They're teammates, not brothers, and it's not as if the Canes aren't offering him an opportunity.
But the point stands that if the Canes cave and trade Fox for nothing, Drury can point to that situation and say "ok, I don't want to sign here, so you should trade me." And if he says it publicly, it even significantly more weakens the Canes position in any future trade negotiation of his rights if Drury makes that claim.
To me, it boils down to this - is it better to lose one asset that you've already resigned yourself to losing but maintain a semblance of respect and credibility in the organization, or lose one trade with a much higher likelihood that you lose a second as well? The only thing worse than making a mistake is making the same mistake twice.