I think this is why they call it baseload. Yes it costs the same, used or unused... but it's always there, ready to be used. This is its strength, not a defect.
> If you really want 100% reliability out of a primarily-solar grid
But no reasonable person wants that. They want "100% reliability, and who cares where it comes from". If you're environmentally minded, you can tack on a "with no carbon"... but even that isn't the same thing as a "primarily solar grid".