The best way to supply "synthetic baseload" with renewables is typically not to use just batteries for storage, but rather a combination of batteries and another storage mode more suited for longer term storage. The latter is optimized to have lower cost per unit of energy storage capacity than batteries, at the expense of lower (perhaps much lower) round trip efficiency.
This latter storage mode isn't needed until fossil fuels are almost entirely eliminated from the grid, since otherwise just use those instead for that long term firming.
The idea that the electricity supply system has to itself shoulder the burden of dealing with intermittency is also mistaken. If it's worthwhile users will be willing to dispatch at least some of their demand. I'm reminded of the argument against deregulation of telecom or airlines: that the new system wouldn't be as reliable or nice. But users were willing to make the tradeoff if the services were cheaper.
