It turns out that some of the attorneys who were designated to argue the case had to recuse themselves for conflicts of interest, which is why the wrong person appeared before the court.

On the one hand, Trump should have waited until after he had a new AG in place before releasing the executive order. Some of the justifications used in the EO didn't make sense, so if Sessions was in place he probably could have helped improve it. Even better, Trump could have met with his administration and ordered the "extreme vetting" without a pause in immigration from the seven countries.

On the other hand, based on the Arizona v. U.S. (2012) I think Trump was right. The federal government sets immigration policy, not the states. Not only that, the states didn't have standing. So I think this decision will be overturned. Footreads is correct that the 9th Circuit Court of Appeals is the most overturned court, so for these reasons it'll get tossed.