"One of the advantages of being more flexible than a 50 foot carnivorous neoreptile is that we should be able to realize we're consuming our resources irresponsibly without having to run out of them first."

Obviously humans have yet to run out of resources.
There are theories that humans have depleted resources in regions and this has had large consequences upon various societies. For example on Easter Island.

Humans can enter a region in which there are endless fat deers that don't realize that this strange creature is predator and therefore the task of getting lunch is easy- for awhile. Humans can and have caused another species to go extinct. Humans can ruin good farming land. Etc.
[But humans can also make unfarmable land, farmable. And they can predict if species could go extinct and take steps to prevent it. And possibly in the future, bring life to a species which went extinct before human existed as a species]

But humans have never actually run out of resources, anywhere at anytime.
People are not starving in this world because of lack of resources.
The problem has never been, that humans lacked resources, but what they have lacked is good leadership.
And/or adequate security to defend themselves against the aggression of other humans. Or, one could say, there are problems of humans interacting, particularly in large groups, in a "rational fashion".

Obviously areas on earth have a finite amount of easily extractable zinc or gold, or whatever. One could say California is depleted in gold due to the Gold Rush.
Or that since the US which was once a major world exporter oil, and now doesn't even produce enough oil for domestic use, it's depleted of oil.

So, humans have constantly consumed most or all of low hanging fruit, but this doesn't mean we have run out of apples, or that we can't grow enough apples tree so everyone could get an apple a day [or a 100 apples per day].

With the US, it has used most of the easy to get oil, and part of the problem with oil production in the US, is we can get cheap oil by importing it.
But there is *probably* more oil in the US which not been mined which we have the technology to mine at say $40 or $50 per barrel, than compared to all we have extracted and used in the last hundred years.

So, as with oil, all resources have more to do with a higher price of a resource then actually running out of them. And dominant factor in something being a higher price, is not the lack of a resource, but it's cost to get it.

In summary, no one knows of any resource that humans will run out of in the next 20 years. And if there was a resource that we could become more expensive such crude oil and say this occurred 4 decades from the present. Then there are other sources of energy available which could be expensive but become cheaper as the technology involved improves, and these are available that allow such abundance that one could push it to at least a century into the future.

And I would say that planning for longer than a century, a could be considered to a bit foolish, and certainly no other animal has such a human luxury.

But if one has an open mind, and will entertain such notions as getting resources from space, one then could have situation of having no end of resources for thousands, millions or billions of years and for human population of trillions or trillions of trillions of people.

But if you wish to exclude using the space environment as a possibility and still wants a plan century into the future. There are other avenues which could resolve one's concerns.

An often cited example is nanotechnology. With nanotechnology one has limitless possibilities. One example is involves Carbon nanotubes, which could vast applications other than exotic uses of bulletproof vest or a space elevator, which could significant lower energy use in a wide array of applications. Carbon nanotubes could a have similar result as the mass and cheap production of steel.

If one doesn't what to put faith into space utilization, or material development, one could look at social organization. Modern cities are efficient in terms of energy use and one could easily make them more efficient.
And for example, such things are the morning commute could become far less common and considered a relic of 20th Century. And as one has higher population densities mass transit can be more easily made more economical and useful for more people more of the time. It's possible cities of 50 million could be common in less than 100 years

Anyways, we aren't going to run out of resources, nor did Mohenjo-Daro run of out resources. But social and economic systems can collapse with the key determining element being poor political leadership.
A
nd though one could say the US political leaderships sucks big time, as compared historically or compared to contemporary nations, it probably easily grades on the curve better than a C.
Though plagues, nuclear war, or such things India having a quantum leap in quality of political leadership, America even with the incompetent Obama and more significantly, the mountain of other fools, will probably continue with involving the end of days- it did after all survive Carter and the 70s, and so it probably will stagger out of the current recession.