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.
