Think about the mechanics of implementing such an emergency declaration.
I agree that declaring and emergency opens a door to a precedent of a lot of really bad things, and I wouldn't want Trump to do it. But declaring an emergency to build a wall isn't going to do much to the public. The vast majority of the public will continue their lives, undisturbed and reading the news.
A declaration of emergency to do a gun grab is a whole different Pandora's Box to open. The outrage and outright rebellion of police forces, state governments and the general population will be one for the ages, and the ability to actually enforce such a declaration will be almost zero. Additionally - and probably most importantly - building a wall isn't challenging a constitutional right.