Yeah a customer charge vs revenue to cover expenses through the rate case are separate here. If everyone suddenly had solar installed, the utility wouldn’t have enough revenue from customers to maintain the current grid without upping that base charge, so now that base charge would be substantial. Since now you aren’t recovering costs from a kWh basis but instead it’s more of a fixed cost to all customers to just maintain the grid.

Let’s say 50% of the customer base installs solar - now the utility must cover the same costs to maintain the grid, yet their revenue is substantially lowered, so the money has to come from somewhere. Do you just increase the rate cost and now the half of customers not using solar are paying double what they were? Well that doesn’t make much sense since anyone with solar is still benefiting from that entire grid.

Perhaps the best solution is to break out electrical bills to generation costs (fuel, maintenance at power plants, wind sites etc. ) and charge those through a rate structure $/kwh, then take the grid maintenance side and just charge that evenly across all households. Many people wouldn’t think this is fair either, but it’s one option.