This is the worst case scenario for immigration but in reality it doesn't work like that. People do return to their home countries at some point in their life not all of them but some of them and it's very documented and they bring invaluable expertise and much needed capital know-how with them. Also, not all migrants are unskilled/low skilled labor, some are very high skilled and can benefit the host country enormously whether directly in taxes or indirectly as rising productivity and national wealth as a whole.