I think you make the strongest case for government investment (which is) in expensive, capital-intensive, high-risk projects; this is probably the most defensible area of government spending. There are two strong counter-arguments I have heard against yours. The first is that the reason private organizations do not invest in these projects is that the government's tax structure makes such ventures unprofitable, and the government crowds-out private investment in these projects. The second argument is that the government is the only investor in these projects because they are investing large sums of money when it is unwise to do so, because the project is not yet worth doing, or it is too risky, and a private organization would have invested less initially, and followed-on as the project became more affordable.