Monopolies (especially via regulatory capture) are bad. Rich folks who actually made the money (not inherited it in fifth generation) are usually pretty okay.
I disagree: What billionaire "tackled" the Manhattan project, or the US interstate highway system?
Even if we exclude governments/taxpayers for some arbitrary reason, that also leaves operations consisting of lots of sub-billionaires.
If it seems like only billionaires can do such things these days, perhaps that indicates that other options have been politically neutered by somebody for some reason.
The interstate highway project was arguably taken during a very deep and long-lasting crisis by the off-the-charts administration of FDR, who served three consecutive terms, despite whatever traditions, and deeply reworked many American state institutions; talk about overconfidence.
I'd say that the peak power of a POTUS is larger than that of a billionaire; the latter may have a longer planning horizon though, and put in effort and resources for a couple of decades.
One example here: https://www.washingtonpost.com/news/answer-sheet/wp/2018/06/...
I disagree too.
SpaceX? - How much work and knowledge of taxpayer funded NASA did SpaceX reuse.
Carnegie? - Public libraries isn't a project. Public libraries were always there. It was nice of him to create them but it is nowhere close to creating interstate highways or the Channel Tunnel or the Golden Gate Bridge.
Judging from the way SpaceX builds, fuels, and flies their rockets and engines, very-very little.
Their contracts with NASA and later DoD did, of course, play a significant role. But I suppose they were chosen on merit.