All forms of energy extraction ultimately convert mass to energy. The coal we burn weighs imperceptibly more than the combustion products. That's really a red h

All forms of energy extraction ultimately convert mass to energy. The coal we burn weighs imperceptibly more than the combustion products. That's really a red herring, though -- you must be able to harness an exothermic reaction to convert what you have into energy, and unifying QM and GR is unlikely to have any implications about reactions at energy densities we can usefully use.