So, TRISO is depending not just on the nuclear island being cheaper, but the non-nuclear part somehow becoming less expensive. This probably involves very high temperature, for example for nuclear air Brayton. But any time you go to higher temperature material problems rapidly accumulate.

In any case, there will always be a cost premium to the nuclear part of a nuclear power plant, if only because it has to be very reliable because it can't be repaired if something serious breaks. In (say) a coal fired power plant one can send workers into the guts of the shut down plant to fix or replace things. In a reactor, I don't think that's possible, even if the fuel has been removed. The residual radioactivity is too high.