Most viable hydro sites are gone, yes. But you don't need a viable hydro site for pumped hydro. What you need is water at the bottom of a hill. That's common.
If you just have water at the bottom of the hill you'll need to build a giant tank on the top of the hill. The best spots for pumped hydro have a natural depression elevated above a water source, so you can use that instead.
(I agree that this is different from traditional hydro locations, and so there are many good spots still available)
The best spots already have a reservoir up top, or most of one. Pumped hydro is so much cheaper than other long term storage options you can spend a little extra on the reservoir and still come out ahead of competitors.
(I agree that this is different from traditional hydro locations, and so there are many good spots still available)