You would effectively have corruption. Governments would be more reluctant to take actions which could be negative when they have direct ownership vs their normal "indirect ownership" via taxation.

> what happens when the US government suddenly owns notable (or even controlling) stakes in companies?

This is effectively similar to nationalization, along with the pros and cons that come with it.

I'd love to see a list of pros. There are no pros. It's effectively death to competitors and actually extremely damaging to the companies themselves as well.

Make those shares non-voting,