In such a case how the Americans could justify the college expense (see my tagline)? Unless they decided to chose the field which does not have marketable application. And even then they will do better studying abroad where education is cheaper.
However the total knowledge base of America would be increasing.
Your understanding is that America is not a nation but a marketplace. Do you think that things like Constitution are product of a marketplace or rather of specific national culture? If the last how America can survive in the free market world without borders?
Having more skilled people around would reduce the cost of education equally to what it would reduce the wages of whatever else. In reality however I dont' believe it would either decrease wages or reduce education costs. Human beings I believe are a net gain, if you let them unleash their creative energies in a free society.
Without immigrant professionals, a lot of our greatest companies would never have been started. For example google was co-founded by an immigrant. If we'd turned him away another nation might have tooken the lead in search engines.
I think the main problem is you are looking at the economy as set in stone in its size and wealth. So more people means the wealth has to be divided among more people. Which is simply wrong.