The key point is that the difference between "wholesale electricity" and "retail electricity" is based on total costs which don't change if you buy 1kWh less from the grid - while it's currently priced "per kWh", it's not really a marginal cost but a fixed one, so people in aggregated would have to pay the same total amount to the grid no matter how much home solar panels are built.

There is a cost advantage currently, but that is a side-effect of how we currently do accounting of grid costs, not reflecting any actual advantage in reducing total costs across the country.