If they can master this, they're at least 10% of the way toward figuring out how to contain high-level radioactive waste! That brilliant (!) engineering marvel needs to be contained at least ten times longer than this clock is supposed to run.
Easy peasy!
"Clean! Safe! Too cheap to meter!"
Fusion power would also be cost-effective enough to literally turn CO2 and water back into hydrocarbons, desalinate ocean water for large-scale irrigation, sustainably produce nitrogen-based fertilizer from the air, and otherwise solve virtually every sustainability problem we face or will conceivably face for centuries. (The rest can be solved by harvesting mineral resources from asteroids instead of the Earth's surface, but that problem can be solved through better orbital infrastructure, which turns into an energy problem.)
I don't see why anyone would charge a flat rate for unlimited electricity when you can pay off the enormous debt faster by charging market rate.
Lots of operational expenses are dominated by energy usage. Cryptocurrency mining would only be the start of it...