There is, in fact, scads of room for add'l pumped hydro. What there isn't room for is more native hydro generation, which needs watershed and a dammed-up river. But pumped hydro only needs an elevated depression. That has actually been a dike around the top of a hill. Natural topography is cheaper, e.g. a box canyon with a dam at the mouth. An alpine pond with no outlet is cheapest of all.

People like to insist pumped hydro is badly limited by geography, but hills are very common, and need not be nearby: transmission lines work very well. Transmission losses matter little when top-line generation has zero opex and minimal capex. You just build a little more of it.