>People take it as an assumption that cronyism will always happen if the government invests too strongly/consistently in a certain thing.

No, it's the opposite. It's a conclusion taken from empirical evidence looking at success rates in the real world. As the US has engaged on its most recent bout of industrial policy, industrial activity has declined, not gone up. Again, this is expected. If you through protectionism make chips and steel in the US inefficiently, everyone in the US using those products as inputs suffers.

There's some limited cases like developing countries engaging in catch up growth , but there's virtually no evidence for effectiveness of these policies in cutting edge technologies, which isn't surprising because they by definition tend to rely on supply chains and knowledge sourced from all across the globe, and to this protectionism is particularly disruptive. And even in developing nations most of the time practices like ISI (import substitution industrialization) fail, it's devastated Latin America in the 1950s to 1980s.

I don't know what USPS proves in this case because it's sustained by a government monopoly. Obviously you could if you wanted privatize mail delivery.