"I just can’t see any circumstances under which steam engines are “coming back” and becoming competitive for electricity no matter how cheap the firebox fuel is."

Industrial grade steam is still widely used and that probably won't ever change except to move from steam to supercritical CO2 and then only for power production. Most steam is used to do other things that are critically important to modern society. The biggest one is to make fertilizer without which we can't feed most of the planet. Your understanding of how industry works is fundamentally flawed.