> antitrust is really simple in my view: companies should principally be in one line of business[0] as that's the most efficient configuration for the economy as a whole. some of those businesses will be larger than others as a natural consequence of the given industry (e.g., capital-intensive businesses like aerospace), but no business should be larger than it needs to be (in the long-term; short-term dynamism is "inefficient" but promotes longer-term optimality).

Would this limit vertical integration? E.g., could SpaceX exist under your scheme, making rocket engines, rockets, and satellites? Is that all one line of business, or is it multiple lines of business? Is Apple's hardware and software business one line of business or two (hardware, and separately software)?

It seems to me that your approach is easier to apply when a company strays from its line of business rather than vertically integrates. So a company that makes hardware, then years later gets into software for its hardware, that might be a problem, but then how could that be a problem if Apple-like companies exist concurrently.

I think all antitrust schemes are tricky and slippery. Not that we shouldn't have an antitrust scheme, but that I'm not sure how to construct one that doesn't somehow suck.