The global socialists fear the wall more than anything. It is like the third ghost in the Christmas Carol...they tremble at the thought.
Once it's up, the push for socialism, excessive costs to house and take care of illegal immigrants, downward pressure on wages, and, future voters of socialism will be decreased significantly.
In short, any National Emergency that follows cannot do as much damage to America as an Open Border does. You will have a more secure America and a push for more changes to your immigration policies so they are based on merit and "American Values".
I say, as emergencies go, this is far more legitimate than any I've seen in my lifetime. Outside of the Cold War, in which the West was always in an emergency situation with the potential of a nuke, If Trump calls this, nobody should doubt it's significance.