Storage does not need any "breakthroughs". It will be built out when there is renewable generating capacity to charge it from. In the meantime, NG plants fill shortfalls.
Globally 16% of electricity is produced by traditional hydro annually that can cover the majority of the projected need for storage in a pure wind/solar grid.
Also, by the time we need significant batteries the costs will have fallen even further. If you want to eventually cover 10% of the grids daily demand from batteries using projected costs from 2030 to 2040+ it doesn’t look unreasonable.
Renewables with straight gas backup and no other storage are already lower carbon than any other option, and batteries and off river PHES have only just started getting cheap.
The breakthroughs we need to cover the final gap have already been made if you're paying any attention at all.
Stop concern trolling
It is also perfectly capable of meeting dispatchable loads like heating, chemical production, and EV charging, and adding them to the grid will bring the ability to meet electricity even higher. Considering the storage and dispatchable low carbon energy that already exists, the remaining part would produce less carbon than would be released by expanding Uranium mining.
There is not enough uranium to meet 50% of world electricity demand using current technology for long enough to wear out a single generation of wind turbines or solar panels.
Your imaginary all nuclear future is both impossoble and worse than the trajectory we are currently on.