> You're not wrong, but crucially those three broadcast networks didn't run their own studios.

Yes, they did (starting before they even were TV networks; NBC Studios has been around since the 1930s), though they also purchased outside content.

Just like the (more than three) major streaming services.

> They couldn't prefer their own, lowly-rated tv shows over more expensive, highly rated shows run by other studios.

Yes, they could. And did.