Why don't batteries scale to grid level? I don't see how this assertion could be justified. Do you mean a specific KIND of battery?

Renewables + storage certainly could provide 100% of the needs for the grid, in a properly designed system. At this point it's just a matter of quibbling over cost. Since the world could be spending upwards of a quadrillion current dollars on energy in the next century there's a hell of a lot of financial headroom to scale things up.