This is probably true, but they should still be researched in case they become useful in other settings, e.g. Mars, or even places on Earth with poor renewable

This is probably true, but they should still be researched in case they become useful in other settings, e.g. Mars, or even places on Earth with poor renewable energy potential A big problem with Mars is that getting fuel there is expensive, they sometimes have large dust storms, and their atmosphere is very thin. Solar power would work fine there 90+% of the time, but outside of fission, there doesn't seem to be any good technology for powering a human habitat over long periods. A rich thorium deposit on Mars could go a long way in creating a self-sustainable habitat. Some thorium reactor designs are notable for their high inherent safety, which is probably a big concern for any human habitat on Mars for morale reasons (think about how much a fatal nuclear disaster on a Martian colony would scare people about space exploration)