Secondly, what are you referring to by "what we have now"? Immigration law in particular, or U.S. policy on a macro level?
It's been changing, sure, but the change you address is different from the change of say dropping a nuclear bomb on all our major cities or discovering sources of unlimited, zero-emission energy. The change I'm talking about, which is eliminating immigration law completely by making ANYONE able to come here is entirely different from the "continuos change since inception" that you're addressing.
By "what we have now" I mean the state of everything as it is today. The economy, the people, our values, our resources. When you increase the population and let ANYONE in, all of these things will change.
Therefore it's worth asking if that "whatever it is" will remain good in the face of any proposed changes.
I'm a descendent of immigrants born to poor circumstances. I'm lucky to be here as a result of getting past strict immigration policies. But I gotta be real, If America tried to "save" every one on the face of the earth by letting everyone in need into this country, we'd probably triple our population, and change the state of everything.
1. Arguments against immigration stem not from moral imperatives, but from survival instinct.
2. If we open up immigration, America as we know it will change dramatically, in either a good or bad way. I think what we have now is already pretty good, so why change the status quo?