Much as i don't want to cause any further depression in an already quite depressing thread, but if you follow Joseph Tainters work on The Collapse of Complex Societies.

https://www.youtube.com/watch?v=G0R09YzyuCI

a) Complexity has killed off pretty much every major society prior to ours.

b) People's innate desire to problem solve increases complexity.

c) Cheap energy has enabled this process.

So there's an argument, an unlimited source of free energy would actually be a major problem because

1) It would enable more complexity, which is already pretty unmanagable.

2) You've still the problem of out of control resource usage.