Energy in aggregate will never be completely free. Nobody will spend billions of dollars for zero return on capital. Energy, if utilized correctly in the optimistic sense, will be a small part of global GDP due to its efficiency. It'll be cheap, but not free. Bitcoin has difficulty adjustments; it doesn't care how expensive or cheap energy is. If fusion becomes commonplace, then miners will be competing with each other in terms of fusion marketshare in regard to mining bitcoin.

They don't produce enough value to maintain their lifestyles, or the lifestyles of their descendants, and thus they become net sellers, passing bitcoin back into circulation. Only someone who produces more than she or he consumes, and thus has an in come, can maintain their current bitcoin stack. Otherwise, they need to start drawing down their assets to pay expenses.