Energy Storage is a solved problem

> Extraordinarily good PHES sites can be found in most regions of the world, with extraordinarily low capital costs

This sentence is doing a lot of heavy lifting. There's plenty of locations where pumped hydro isn't practical (land too flat, too seismically active, too dry, too ecologically sensitive whatever). Just like energy generation has to be reasonably close to usage before grid connectivity becomes too expensive and losses to heavy, storage has to be reasonably close to both the generation and usage.

Right now, pumped storage doesn't have to be that close because it's just covering a bit of headroom in capacity and so can just have it in one area, and thus generation on the outskirts of that area will flow towards other areas instead, obviating the need for the pumped storage to have grid capacity to everywhere. But if we really have high wind/solar usage pumped storage will have to carry base-load and so this becomes more critical.

To really believe that pumped storage is some kind of universal solution, I'd really like to see characterization of what distances it is feasible to build the required grid capacity to, and if there's enough density of viable sites to cover all major population areas at that distances. A handwave about "found in most regions of the world" is not enough.

Alternately:

Large scale storage is reasonably well solved.