I think 18F going private is a terrible idea. The US government should directly employ the people that are responsible for building and owning its core infrastructure.
This is already untrue, leaving 18F aside - lots of core infrastructure is outsourced and the vast agencies are mostly operating or maintaining things. Plus where does that road end - they don’t build their own operating system after all. They have to outsource some of it.
I would argue that companies like Plantir show that it's actually quite viable to be in the private sector while simultaneously serving the public sector. Granted Plantir is primarily military and law enforcement but I would say the overall model could work well for other parts of the govt.
It's important to have some in house expertise to know what to buy from the private sector though.