I will attempt to explain this seemingly strange comment about falling world populations.
Elon Musk is
very much an optimist, but not a climate optimist (otherwise why would he have made Tesla/Solar City?). As such, he likely believes we will be able to produce ten-fold
sustainable improvements in farming efficiency (i.e. through in vitro meat and gene editing of plants). He is also very much a believer in capitalism as he has skyrocketed from a South African nobody to the world's most important visionary, and why wouldn't you after such an experience?
And unlike some people have said on this topic, the carrying capacity of Earth for humanity is not a fixed number, and never was the second we began innovating. It started rising as soon as we began intentional ecosystem modification and has continued rising ever since, although currently long-term carrying capacity is far below the population due to unsustainable farming practices.
If carrying capacity was fixed, of our
7,516,913,663 people, about 7.506 billion of us would be doomed to die from famine and starvation rather imminently, with perhaps 10 million making it out on the other side.
I do, however, highly doubt that a falling population will ever be a problem, because
1) advancements in genetics and biology will likely allow us to grow infants entirely outside the womb in 10-25 years time
2) a growing population is only necessary to sustain an exploitative profit-based capitalism
And with that, I went and checked his Twitter feed and the culprit tweet seems conspicuously absent