> US government funding of science isn’t a net cost due to taxes on the long term economic productivity that results.

There's an assumption here that deserves some closer examination. If we are taking this as a justification for federal science spending, we would have to also support a policy of awarding research grants on the basis of expected long term return on investment, which is not the criteria applied now. Furthermore, we would have to justify this spending in competition with whatever economic investments the government could make elsewhere, or that the American taxpayers would make if we let them keep their money in the first place. From the standpoint of scientific research I don't think this is necessarily what we want, but even if it was we would have some hard questions about the last few decades of federal research funding.