They generated economic activity while they were in the US, no? That seems to be a net positive. You'd otherwise have to be able to argue that, if you replaced them with a US citizen during the time they were here, the greater economic activity would have been generated.

If you gave me a million dollars for a piece of paper and I gave you a million dollars for a different piece of paper we just generated $2 million in economic activity. It seems like a stretch to call it a net positive. Economic activity is a poor measure.

Please keep in mind I am specifically asking about what the OP wrote, not about immigration in general.