How do you see the U.S. without illegal immigrants?
I don't see America as being a fraction of what it is today. People tend to forget that illegal immigrants provide a source of capital for the country, provided the foundation of this country (even our forefathers were "illegal"), and usually work the jobs that no one else would want to work---for lower wages.
I don't see that as a realistic scenario, unless we allow for free immigration. But, I think the result would be bad to the economy. Studies repeatedly show a net economic benefit from illegal immigration. It hurts a few, but benefits many.