So who is that is mandating a single power source? Market forces are deciding that coal and nuke are too expensive to solely rely on. Solar prices continue in free fall. Wind at scale has bean counters taking a second look. Pumped hydro is cheap and great, depending on location. Plain old hydro is also great but limited in where it can be implemented. The civilization of America followed the rivers west and that was followed closely by rail that pretty much followed the contours of the waterways because of the more gradual slope offered by the river valleys. So now, we find ourselves settled and developed along the flood plains where building any new dams would mean massive displacement of developed towns and cities. That is very unlikely to happen so we need to find alternate ways of meeting our power demands and also better ways to transport our power from coast to coast.