You're wrong about that. Back up with green hydrogen is quite plausible. Europe has enormous salt formations in which cavities can be solution mined for gas storage (this is one of the chief ways natural gas is stored). Storing hydrogen, the cost of these caverns per unit of storage capacity in these would be about $1/kWh. The total potential capacity there is in the petawatt hours, far more than would be needed.
A combined cycle power plant costs about $1/W of capacity (and for rare events, simple cycle would be even cheaper), so one could back up the entire grid with these at a small capital cost compared to powering the grid with nuclear. For Europe, these would also be useful for seasonal leveling, allowing solar to provide a larger fraction of Europe's energy demand.
Hydrogen is an example of "Power to X" (PtX), where excess power, when available, is used to make some very storable commodity. This review article talks about how important these are to reaching 100% RE.
https://ieeexplore.ieee.org/document/9837910
"With every iteration in the research and with every technological breakthrough in these areas, 100% RE systems become increasingly viable. Even former critics must admit that adding e-fuels through PtX makes 100% RE possible at costs similar to fossil fuels."