I also thought manned space was the solution to limits to growth...but that was in the early seventies....

Then came human ingenuity and some facts.

If there is an expected shortage of energy, there are several ways to cope with it: reduce consumption (in Europe, per person, we use 3 times less energy than in the US), diversify the sources (sometimes using wind, sun, tides, geothermal sources, nuclear energy, much better isolation against temperatures, noise, pollution).

The human population isn't growing as fast as we thought it would and if there's a shortage, immigration will be welcome.

Mankind doesn't need more and more and more energy...Why would it and for what? Cities are being changed in order to keep cars away from their centers. Yes, some parts of the world like China and India are using more and more energy but they also realize what it means for their health.

It's a matter of spreading means, needs and consumption in an acceptable way.

Now, even if we do ever need other worlds for mankind, it doesn't at all mean humans are also needed out there. Robots will be doing the job (they don't breathe nor are needed back on Earth). Private companies are always looking for the cheapest investment.

This is why I think using the Moon for economical reasons (raw material and letting the private industry do it) in order to get humans back to its surface, isn't a great argument.

Because the finality isn't about science, strategy or even economy, the finality is (for Zubrin and others) to have humans out there.