As the Texas power outage demonstrated, the US grid is not monolithic. Having more interconnected regional grids actually makes it easier to get individual regions to high renewable percentages while allowing the market to continue to mature lower cost storage. Storage is slightly behind the solar manufacturing S-curve in terms of dropping costs, but once a number of EV supply plants start ramping up, I think that cost optimization will strongly drop storage costs.

Is there a reason to believe that a solution that works on the scale of a reasonably sized country suddenly stops working on the scale of a tight federation of states or comparable size?