I voted "yes" mainly because I agree with others that, in theory, no player should be considered untradeable. Hell, the Oilers even won a Cup after trading Gretzky.
However, JR/PA should only trade Horvat if it's a necessary step to making the Canucks a legit contender for the Cup. I figure it's going to take at least 2 more seasons after this one to dig the Canucks out of the deep hole Benning dug. By that time, Horvat will be 29 and Miller will be 31. I like both players, but age related decline is a real thing and there's no reason to believe they'd be exceptions to that curve.
In other words, by the time the Canucks are good enough to have Cup aspirations, Bo and JT are likely too old to be indispensable to those aspirations. What the Canucks will need to compete for a decent number of seasons are quality, young players on ELC/RFA contracts to complement the more expensive core players. Trading players like Horvat and Miller are probably the best way at this point to getting that cheap, young talent, given the barren wasteland left behind by Benning.
The only players I would not want to see traded are Hughes, Demko, and Pettersson (in order of priority). In the case of Hughes and Demko, I think it would be next to impossible to get better value given their age and what they've shown themselves to be capable of. For Pettersson, as disappointing as this season has been, it's too soon to give up on him.
I'd say it's important to determine what BH is looking for in his next contract, since there's a good chance it will be his last big contract. I have a hard time seeing the Canucks competing for a Cup if BH and BB are the two most expensive forwards on the team.