> they'd be no worse off than today.

They will, assuming the people that went off grid stop paying for it. As fewer people pay for it the costs per capita grow

The cost of the grid has already been paid for. Upgrades to the grid has a higher per-capita cost, if there's fewer people paying for those upgrades today.

But they're not worse off, because the upgrades are better. For them to be worse off, the upgrades they pay for has to be worse than what they got today.