I think you and goobster are saying pretty much the same thing here. You're completely right that the grid is the storage mechanism, at least today. With demand inflexible and wind and solar inflexible, as long as the flexible parts of the grid can make up the differences, everything works. That hits goobster's point about transmitting energy over longer and longer distances. When one part of the grid becomes saturated, whether it's one neighborhood or a group of states, the grid has to be expanded to pull in more flexible resources. HVDC is awesome for that, but it's also pretty expensive. Ballpark estimates are like $150M per converter station and $2M/mile of line. Projects quickly reach billions of dollars.

Any transmission line also has public opposition. They cross rivers and backyards. While some impacted don't mind and don't mind the payment that comes with, some will fight it to the bitter end.

I'd say the technical aspects are pretty well understood. It wouldn't be hard to design a system around mostly wind and solar, but nobody would be willing to pay for it.