I think costs would go down if the excess power was not fed in to the grid. Reason is the Rooftop is now displacing the Utility Solar and Wind and they are reportedly the cheapest. The curtailment reduces the capacity factor which increases the cost and destroys the economics of the Wind and Solar. Also the grid is becoming unstable and requires higher frequency management costs, transmission etc which are then recovered over smaller grid volume.
Imo the reason it is not discussed is the people with Rooftop vote whereas the wind and utilities don't so it is easier to ignore the problems caused by rooftop because it is popular. The renewable industry is also more interested in building stuff than how effective it works so at the moment they are not doing the critical analysis or risk assessments which would have avoided the problems now appearing.
As more people and businesses avoid contributing to the network and transmission costs by installing their own rooftop then the problem gets bigger for the utility wind and solar, it just takes a bit more time. Eventually the modelling will catch up with reality.
