Continuing to operate their existing reactors would certainly have been cheaper for Japan. So the decision to replace them with fossil fuels (now LNG + CC, not the fossil fuels of decades ago) wasn't driven by economics.
After 2011, nuclear plants in Japan need to be audited that means plants must be stopped near the future. It causes massive power supply crisis so power companies built power plant as fast as possible. IIRC LNG power plant is said fastest plant to build and start operating.
Now that nuclear power is fallen out of favor, they went back to fossil fuels, instead of renewables, because it's cheaper for them.
So apparently the advances in renewables didn't just make renewables cheaper: it also made nuclear more expensive than fossil fuels!
Or, maybe, nuclear is now considered "more expensive" largely thanks to the huge negative PR.