Energy storage does add a cost, but it's not prohibitive. It's generally a couple cents per kilowatt hour, give or take.
You strongly need a citation there. The crux of the argument about energy storage is cost. Many energy storage systems have long term replacement requirements. In the case of batteries, they will on average need replacing every 7 years. Batteries are expensive. The battery buffer system just doesn't make economic sense in the long run. Hydro-turbines could potentially be cheap to maintain for 20 or more years, so I think you should clarify the type of energy storage system you are talking about.