To make solar power viable, we need a solution for overnight energy storage.
Batteries are complicated.
Do you know what isn't? Water go up. 
To make solar power viable, we need a solution for overnight energy storage.
Batteries are complicated.
Do you know what isn't? Water go up.
It's good. Like hydro power, the viability is going to be highly site-specific. But it's a bunch of well known parts, so if some geological engineers say a particular pumped hydro installation makes sense, I'm going to trust them.
I think battery and synthetic fuel technologies will continue to improve, and the range of places where pumped hydro is the better choice will shrink over time. But in the best sites, I expect it's probably going to stay the most efficient choice for a very long time, the same way the biggest hydro power plants dwarf the biggest nuclear plants.
Why would the range of places shrink over time?
Because the alternatives get better.
What alternatives at that scale?
Batteries and synthetic fuels?
Did you read the post you're replying to?
I find reading interferes with the posting
I don't think batteries are designed for the duration of storage you want a backup of pumped hydro for, and it will remain to be seen what a synthetic fuel industry looks like (and whether it would ever be cost competitive with pumped hydro at the places where pumped hydro is currently viable).