I know this is a new mechanic for D4, so there’s a lot more to work through than other aspects.
To start, I’m not sure that a State Industry should necessarily completely kill the private sector. Particularly if it’s an open market (e.g. Energy, or Telecoms). If the government subsidises theirs massively, that will undercut the private sector, killing it in the process. However, if they are merely breaking even, or even trying to make a profit, private companies will be able to compete, and have their own versions. This is of course excluding trains and water which are natural monopolies; 100% nationalising these will make competition impossible.
I think that if State Energy income is <0, Private Energy should be reduced but not canceled completely. It should exponentially die out as State investment increases though. For really high State industry values, I’d even add an effect on Corporate Exodus due to the anti-competitive behaviour such subsidies engender.
The cost and income of implementation should be contingent not only on GDP but also the size of the state version. I.e. If GDP = 0.8, the player would get more income for a State industry = 1 than if the industry was = 0.5. Same goes for setting up a state run sector. The implementation cost should depend on how big the player wants the industry to be, not just GDP. FWIW, I’d also include Health and Schools here. Privatising Healthcare wouldn’t just see a load of hospitals being empty instantly, but be taken over by private firms, who may then decide to close them depending on GDP.
That kind of my point more generally actually, the state industries should act more like State Healthcare and Schools than they currently do.
I’d also include a short decaying drop in unemployment when setting up new state industries to account for the new buildings and infrastructure that would be needed to be built.