The twelve-hour-war theory helped erase reserves

Did the belief that nuclear war would end within twelve hours contribute to the disappearance of reserve fleets?

Yes; if war would be over before mobilization, reserves appeared unnecessary, but removing them also removed mass when that assumption proved false.

The theory of a short nuclear war provided a justification for disposing of reserve ships and organizations. Under that model, a fleet mobilized after the opening exchange would arrive too late to matter. Actual conflicts did not conform to a universal twelve-hour timetable. Once reserve capacity had gone, navies had also lost the ships, personnel, and planning habits that made quantity available for a sustained war.