Why, you can still be useful but only marginally so.
For instance the demand for ESL classes is largely driven by school curriculum, which in China is set by the government rather than organic market forces. Welding, on the other hand, is necessary for physical production of certain goods. It can be argued about second order effects from learning a language but from purely libertarian economic POV a welder creates more added value.
All those immigrant tech workers in the US are assuredly adding more to the economy than they are taking. (And yes, they are immigrants on the path to US citizenship, not expats.) Your argument makes no sense.
