Demand shifting is just a long winded way of saying, "we can't generate enough electricity, you just need to stop using power at certain times of day." And it's easier said than done. Residential demand shifting, and some forms of industry (e.g. arc furnaces) can shift demand. But others cannot: the pumps powering your sewage system needs to work all day around. Same with data centers, and plenty of other industries.

Right, but we don't need to shift all demand for it to have a big impact on the amount of storage required. Any demand shifted is energy that doesn't need to be stored.