I do not get where you think that need of a fossil grid comes from? Seems like a hyperbole based on preconceived notions? Research has consistently shown that a 100% renewable grid is feasible and economically sound.

> Much of the resistance towards 100% Renewable Energy (RE) systems in the literature seems to come from the a-priori assumption that an energy system based on solar and wind is impossible since these energy sources are variable. Critics of 100% RE systems like to contrast solar and wind with ’firm’ energy sources like nuclear and fossil fuels (often combined with CCS) that bring their own storage. This is the key point made in some already mentioned reactions, such as those by Clack et al. [225], Trainer [226], Heard et al. [227] Jenkins et al. [228], and Caldeira et al. [275], [276].

> However, while it is true that keeping a system with variable sources stable is more complex, a range of strategies can be employed that are often ignored or underutilized in critical studies: oversizing solar and wind capacities; strengthening interconnections [68], [82], [132], [143], [277], [278]; demand response [279], [172], e.g. smart electric vehicles charging using delayed charging or delivering energy back to the electricity grid via vehicle-to-grid [181], [280]–[282]; storage (battery, compressed air, pumped hydro)[40]–[43], [46], [83], [140], [142], such as stationary batteries; sector coupling [16], [39], [90]–[92], [97], [132], [216], e.g. optimizing the interaction between electricity, heat, transport, and industry; power-to-X [39], [106], [134], [176], e.g. producing hydrogen at moments when there is abundant energy; et cetera. Using all these strategies effectively to mitigate variability is where much of the cutting-edge development of 100% RE scenarios takes place.

> 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. These critics are still questioning whether 100% RE is the cheapest solution but no longer claim it would be unfeasible or prohibitively expensive. Variability, especially short term, has many mitigation options, and energy system studies are increasingly capturing these in their 100% RE scenarios.

https://ieeexplore.ieee.org/document/9837910

Or we can take a look at Wikipedia for an even broader view

> 100% renewable energy means getting all energy from renewable resources. The endeavor to use 100% renewable energy for electricity, heating, cooling and transport is motivated by climate change, pollution and other environmental issues, as well as economic and energy security concerns.

> Research into this topic is fairly new, with very few studies published before 2009, but has gained increasing attention in recent years. The majority of studies show that a global transition to 100% renewable energy across all sectors – power, heat, transport and desalination – is feasible and economically viable.[5][6][7][8] A cross-sectoral, holistic approach is seen as an important feature of 100% renewable energy systems and is based on the assumption "that the best solutions can be found only if one focuses on the synergies between the sectors" of the energy system such as electricity, heat, transport or industry.[9]

https://en.wikipedia.org/wiki/100%25_renewable_energy