I think the wall should be built, ocean to ocean... and it's immoral not to do so.* However, Trump failed to control the narrative around the shutdown, failed to accept a reasonable deal a year around, and squandered 2 years of a friendly house/senate. It is NOT legitimate to call it an emergency now... especially for the precident it will set, as Nero explained.
*if you buy into the narrative that law enforcement is inherently racist/biased or whatever, any scenario where they can cross freely and subsequently are tracked down with drones or intercepted by patrols just creates opportunities for that racism to play out. A passive wall, on the other hand, will make it harder for bad actors (there are some, and you're intellectually dishonest to deny it) from passing through freely; it will make it easier to detect and foil where they are getting through the wall; and it will direct legitimate people to ports of entry and eliminate the viability of these coyotes to escort people across illegally while sexually assaulting 30% of the women who hire them.