I think that resource constraints and environmental damage are real, but I don't think they're the cause of collapse.

It seems to me that complexity itself is the cause of collapse. The problem with human societies is that there's no mechanism for reducing complexity. Each feature of a complex society has a stakeholder, someone who would lose out if that feature were reduced or abandoned. So societies grow in complexity without bound until the complexity collapses under its own weight.

A similar effect is very familiar to every engineer, especially in software. Systems grow in complexity until they are unmaintainable. Then you have to trash them and start over. Removing complexity is difficult, because there is a "reason" for every piece of complexity you have.

I don't think inventing Mr. Fusion would stop this cycle.