>you wouldn't need to build the destructive supplemental power supplies in the first place.

Yes you would, just less of them. A major manufacturing plant isn't going to be able to fully supply it's power with just solar on its roof tops. It would need a massive solar farm plus a lot of battery storage. You might be able to supply, and even oversupply, your house most of the year. And when you are generating more than you need you can help supply your neighbors. But you aren't going to be able move that excess generation very far. Like anything, distance causes losses. That is why we have giant transformers on one end to transmit electricity at high voltage and low current, then switch it back to low voltages and higher current for actual use.