This is falling into the same trap. Fusion power is not a panacea. The Earth has a heat budget, where it can only emit so much heat over time (moderated by several factors, e.g. the current crisis with greenhouse gases is that we are lowering the rate at which heat is emitted). You cannot simply build an infinite number of fusion generators to generate an infinite amount of energy without
also generating an infinite amount of heat that then gets dumped into the Earth's environment, killing all life in the process.
The whole point is that eventually a planetary population must stop producing more energy. Those that don't, die. At best, you could try to colonize more planets to increase your heat budget. The open question, then, is whether or not interplanetary colonization becomes feasible before or after the point at which energy production becomes do-or-die (or rather, don't-or-die). There is no law of the universe that says it must be so.
