> So say you're an ISP deciding whether to invest the same money into building fiber out to one of two areas, one where you'll make 1 million dollars a year and one where you'll take 2 million a year, and you make the rational decision to build out to the more profitable area.

Maybe those two areas should be served by two different ISPs. If an ISP is operating in such a large area that they can't afford to keep all of them reasonably up to date then maybe that's a sign that the ISP is too big.