Some key insights from the article:
Basically, what they did was to look at how much batteries would be needed in a given area to provide constant power supply at least 97% of the time, and the calculate the costs of that solar+battery setup compared to coal and nuclear.
I mean there are ongoing costs with any form of power generation. Obviously there's fuel costs for most, but even other renewables have maintenance costs. You'll also need to keep investing anyway as power demands increase over time. So newer solar installations eventually replace the old.