Existing solution is hydro storage. It's massive infrastructure and there is a long way to go.

Fusion power plants are also being developed. How is that an argument for anything?

Failing to provide details smells a lot like „wonder weapons will turn the war around“. There is currently no battery tech that is remotely economical, and other more obscure solutions haven’t made a lot of impact either, presumably for economic reasons too.

There isn't a lot of storage because there isn't a lot of surplus energy to store right now. It's more economical to build more generation capacity. But storage is really not hard: just store hydrogen. No magic technology is needed. You can buy Megawatt range electrolyzers today.