The thing about SpaceX and NASA is that their purposes aren’t really the same. NASA does space exploration and science type work to a large extent, which requires them getting their equipment up into space, but since rocket launches themselves are no longer the frontier of space technology, they don’t really want to be in charge of launching everything up the that people want launched (which is a lot these days), they want to focus their efforts on pushing the boundary. It would be like trying to solve the problems with Boeing by making the FAA build all the country’s planes instead. Not to say that nationalization is necessarily bad, but more that if it were done, it would make more sense to keep SpaceX it’s own entity as a state run corporation than to fold it’s commercial rocket launching into NASA.

Alternatively, something else I could imagine threatening Musk with, if the government had the stomach for it, would be to seize SpaceX and then make it employee-owned, which avoids changing it’s competitiveness in the launch industry (it has become so dominant because, for the moment, it has actually done a pretty good job at reducing launch costs and improving rocket technology, and doing anything too disruptive with it while it remains in that position might disrupt that), but takes away Elon’s share of the money and decision making.