That equivalent inertia can only be done for short periods but that's exactly what grids need in stability - there's generally no lack of total generation, just a need to jump in and smooth out spikes.
You can't build a dam for that price, nor could you do it in under 100 days from contract signing as that battery was built. Batteries are definitely the answer here. The 'more spinning mass' answers don't make sense since Australia literally solved the above problem in a much cheaper way already.
Yep, this is the issue. That and land cost. Also pumped hydro is most useful when you need very large capacity storage, whereas for preventing blackouts you need very high capacity fast generating to fix oscillations or to allow more generating capacity to come on line. They are basically acting as decoupling capacitors (except for AC) in this application.
You need two sufficiently large bodies of water close to each other at different elevations. You don't necessarily need two dams - for instance, the Ludington pumped storage plant adjacent to Lake Michigan uses the lake as the lower body.
Most economically-suitable locations for pumped hydro have been built out already.
You can always use a ton more concrete and force new locations, but the best locations have already been utilized and scaling law of batteries has brought them to the point where they're more competitive than new hydro for this kind of use.
Most economical locations for hydro generation have been built out already. Pumped hydro doesn't require flow like generation does, so there are thousands of times more suitable locations. Those haven't been built out.
In other words, there are lots of places they could build, but it's just too expensive to build and/or the value of long term electricity storage is too low.
