Depopulation of the World - Buddha's Explanation

This requires a thorough understanding of what "yakkha" and "amanussa" are, which is difficult to share here.

In the sutta's words, "yakkhā vāḷe amanusse ossajjanti..." which may literally read "yakkhas let out/release cunning/predatory non-humans."

This could mean several things. If interpreted spiritually, it may refer to demons causing a ruckus by taking advantage of human weakness, thus causing mental illness, destructive behavior, etc.

From what I understand, "yakkha" and "amanussa" may also describe human beings with certain non-human qualities. In this case, these qualities would be quite alarming, possibly refering to those most evil among us. Consider how we migh call an evil person a "demon" or "inhuman", a title usually reserved for the most vile of humans, such as those who take great pleasure in causing pain and destruction, cannibals, head hunters, serial rapists, world-dominating mad scientists, etc. These evil yakkhas then would function as the ring leaders of the less self-controlled psychotic amanussa. You see this trope in popular media and story telling everywhere, such as when the bad guys take control of society with an army of baddies.

This properly belongs under your comment above. Or you could ask a separate question and give this answer.

In the early records, yakkha as an appellative is, like nāga, anything but depreciative. Not only is Sakka so called (M. i, 252), http://buddhadust.net/dhamma-vinaya/pali/mn/mn.037.pali.bd.htm#pg252 but the Buddha himself is so referred to, in poetic diction (M. i, 386).

http://buddhadust.net/dhamma-vinaya/pali/mn/mn.056.pali.bd.htm#pg386 We have seen Kakudha, son of the gods, so addressed (II, 2, § 8); http://buddhadust.net/dhamma-vinaya/pts/sn/01_sagv/sn01.02.011-020.rhyc.pts.htm#sn.1.2.18 and in D. ii, 170 (Dialogues, ii, 200), http://buddhadust.net/dhamma-vinaya/pts/dn/dn.17.rhyt.pts.htm#pg200 the city of the gods, Ālakamandā, is described as 'crowded with Yakkhas' ('gods'). They have a deva's supernormal powers, and are capable of putting very pertinent problems in metaphysic and ethics. But they were decadent divinities, degraded in the later era, when the stories to the Jātaka verses were set down, to the status of red-eyed cannibal ogres. Cf. the older and newer view together in Pss. of the Brethren, p. 245. http://buddhadust.net/dhamma-vinaya/pts/kd/thag/thag.232.rhyc.pts.htm

PED: Name of certain non-human beings, as spirits, ogres, dryads, ghosts, spooks. Their usual epithet and category of being is amanussa, i.e. not a human being (but not a sublime god either); a being half deified and of great power as regards influencing people (partly helping , partly hurting). They range in appearance immediately above the Petas (ghosts) ... They correspond to our "genii" or fairies of the fairy-tales and show all their qualities ... Historically they are remnants of an ancient demonology and of considerable folkloristic interest, and in them old animistic beliefs are incorporated and as they represent creatures of the wilds and forests, some of them based on ethnological features.)

The Arab singular (whence the French "génie"); fem. Jinniyah; the Div and Rakshah of old Guebre-land and the "Rakshasa,' or "Yaksha," of Hinduism. It would be interesting to trace the evident connection, by no means "accidental," of "Jinn" with the "Genius" who came to the Romans through the Asiatic Etruscans, and whose name I cannot derive from "gignomai" or "genitus." He was unknown to the Greeks, who has the Daimon, a family which separated, like the Jinn and the Genius, into two categories, the good (Agatho-dæmons) and the bad (Kako-dæmons). We know nothing concerning the status of the Jinn amongst the pre-Moslemitic or pagan Arabs: the Moslems made him a supernatural anthropoid being, created of subtile fire (Koran, chapts. xv. 27; lv.14), not of earth like man, propagating his kind, ruled by mighty kings, the last being Ján bin Ján, missionarised by Prophets and subject to death and Judgment. From the same root are "Junún" = madness (i.e., possession or obsession by the Jinn) and "Majnún" = madman. According to R. Jeremiah bin Eliazar in Psalm xli. 5, Adam was excommunicated for one hundred and thirty years, during which he begat children in his own image (Gen. v. 3) and these were Mazikeen or Shedeem - Jinns. — The Book of the Thousand Nights and a Night, Volume 1, pg 10, translated by Richard F. Burton, Printed by The Burton Club for Private Subscribers only, 1885. Image from same source.

See also SN 1.10.1, n.1

AN 3.56: Paloka Suttaɱ

Woodward translation:
The Rich Man
http://buddhadust.net/dhamma-vinaya/pts/an/03_threes/an03.056.wood.pts.htm

A wealthy brahmin has heard the story told by the ancients of a time when this world was densely populated and villages and towns and cities grew up right next to each other. He asks Gotama why it is that this is no longer the case and is told that it is because at this time the people are obsessed with lusts, depravities and wrong views resulting in a number of calamaties that depopulate the earth.

A tough sutta for modern man to digest for several reasons. Nobody I think will argue that the people today are not as depraved as described, and that that results in wars, but that this depravity affects the weather and crops and causes the unleashing of demonic beings will be harder to see. And then there is the issue of feeding such a dense population ... and this was speaking of a time when the lifespan of man was in the tens of thousands of years.

How can this be seen?

This world is a work of the imagination, individual and collective. The individual has no way to know the nature of 'the real world', what he can know is simply the three experiences through the senses, what he 'sees' and 'hears' and 'tastes' and 'touches' has no more concrete reality to it than a dream, but because man desires to live collectively he agrees to believe the world is of a certain nature, possesses certain properties beyond solidity, liquidity, heat and light, motion, space and consciousness. Thus the apparent world is one which is arrived at by consensus*. That consensus changes over time.

That's how.

Today we have heard that because of wide-spread lockdowns limiting human activity, the surface of the globe has experienced a reduced vibration. The poles of the earth are moving. There is a huge increase in seismic activity in places where there has been none in centuries. There is global warming and the rise in sea-level. Fires rage uncontrolled destroying millions of acres of vegetation. The US has experienced flooding that has seriously compromised food supplies. There is even a plague of locusts! As for demonic beings, put aside pre-conceptions as to how these beings look and you may see them in the actions of human-form monsters. We have school shootings, mass shootings, terrorists, assassans, child-women-human trafficers... the list of monsterous activities by so-called human beings is endless.

*Actually examined closely it will be seen that outside the sangha there are hardly two people in the world that agree with each other about almost anything. What it is is a 'consensis reality' arrived at by the hoodwinking of weak, ignorant majorites by powerful ignorant minorities.