Basically we still need to do x10 or more in electricity storage (hard to do when you already built all the reversible damns you can build in your country) in most places by still having the risk of a several day grid cut if we don't have wind and enough sun for a while.
In France for example, we don't have space to build reversible damns anymore. Which represent a huge majority of the electricity stored (batteries are only 2%). We can basically store 14h of the country consuption. Handling those "no wind, no sun" cases would need up to weeks of storage.
Moving to renewable energy is basically stressing the grid. The Germans can use the other countries to limit the impacts but it doesn't work when everyone is doing it.
Except if our society is ok to have multiple times per year global power cuts of several days.
I'd also like to stress that CO2 emission are also massively coming from transports and buildings. Focusing massively on electricity generation decarbonation only is I think a nonsense.
Reversible damns are for small amount of energy and short term... but most classical damns do store much more energy - that is an energy you can decide to use now, or in two weeks, or sometime in few months. They store energy, just can't store already produced electricity
Energy efficiency and demand-response have HUGE potential to reduce energy consumption, and to make energy consumption "programmable"
A good mix of different renewable energy (hydro, biomass, onshore wind, off-shore wind, solar) make it impossible to have a "no wind, no sun" case, even if there will be fluctuation in what level of electricity you can produce
Human stupidity.
I prefer to use good ol' technologies such as power plants/damns, they are reliable, they can be done locally (most of solar panels are coming from China), they can be repaired and we can handle them country wise easily.
If we want to reduce our CO2 emissions, we move to gaz (coal -> gaz is -20%), we reduce our consumption (building isolation is -30%, sometimes -50%), and we massively reduce cars usage (by developing trains, another good ol' technology that works really well).
But if we need to reduce our CO2 emissions really fast it's one of the most efficient way to cut 1/5 of the emissions without massively changing how the grid and the other things are working.
But for sure in the end it's more a way to save time and to reduce coal usage that is also producing serious health issues to the populations.