"Legal immigrants have always been chosen by criteria that made it very likely they would never be a burden on the economy or taxpayer".
Always? The Werner von Braun's and other scientists, professors, etc. are the exception rather than the rule. We have accepted many of far less means and capability but that was also at a time when we needed manual labor to settle, develop, defend, and exploit the land and vast natural resources.
Times have changed... We are a welfare state and no longer have an unlimited need for manual labor. What someone produces must be weighed against what they cost and in a welfare state and that makes an unskilled under-educated worker a burden rather than a benefit. Immigrants are supposed to be vetted against the public charge rule, but in practice -- especially under this administration -- it has been completely ignored.