I suspect that the Canucks have decided that McWard is a better choice at RD than Juulsen, but think if they keep him up they're using the wrong criteria for choosing their team.
I'm high on him but really hope the Canucks send him down-if not for Game 1 then soon after. Keeping, for example, the 8th best defenceman ahead of the 9th best defenceman, if that is what the Canucks would think they're doing, makes very little difference to how the team will do in the short term and could potentially make a big difference in his usefulness in the long term.
McWard getting lots of playing time against professionals at a level at which he can be good could, if it went as well as people hope for, turn him into a useful, maybe even very competent, NHL RD. Maybe that's being overly hopeful but imo keeping a young player to play over his head instead when he's not too good to play at the next level down risks decreasing his confidence and increasing his confusion, even just making him a fraction of a second more hesitant for fear of mistake. It doesn't take much hesitation to be making decisions too slowly at the NHL level.
Some may think the risk small, but even if small it isn't one I think worth taking. I'm in favour of sending him down until he's pretty clearly ready. Keeping him could work but there's always that risk, which some consider small but others not so much, that it can hurt in the long run.
There's little to gain in the short term by keeping him up and even if the chance of loss isn't great, it isn't worth it. If he is dominant in the AHL he can always come back up.
Unlike some, I'm ok with him having been given the run this preseason, which for a young player can be good for him. The time has come, though, to give him time to get used to the pro pace while being a good AHL defenceman instead of a marginal (or potentially worse) NHL defenceman.
Those who think of Tanev should note that Tanev spent his first three seasons splitting time fairly evenly between the AHL and NHL.