How could humans have lived 80 000 years in the past? And how could we live 10 years in the future, with people reaching sexual maturity at five, when life expectancy is constantly increasing?
I have never read the sutta. Bhikkhu Sujato wrote:
The Dīgha contains truly mythic texts in DN 26 Cakkavattisīhanāda and DN 27 Aggañña. These set forth a myth of origins, replacing conventional creation mythology with an evolutionary account of how the world came to be the way it is. In these stories, human choices play a critical role in how the environment evolves, and in how it will all fall apart. The Aggañña depicts climate change quite explicitly, showing how human activity affects the plants, the weather, and the natural ecosystem of which we are a part (see also AN 3.56).
The mythology is essentially cyclic. There is no absolute beginning; just another turning of the wheel. Thus even when the world falls apart and civilization collapses, there will be a new renaissance, far in the future, and ultimately another Buddha will arise. He is named as Metteyya (Sanskrit: Maitreya) who in the early texts appears only in DN 26 Cakkavattisīhanāda. He went on to become one of the most important figures in Mahāyāna Buddhism, and many Buddhists even today still await his coming with hope. Yet DN 26 is not taught in order to encourage devotees to dedicate themselves to Metteyya, but to illustrate the impermanence and uncertainty of our lives. The Buddha always taught that we should practice as best we can to understand the Dhamma in this life.
Life expectancy in the human realm (collectively all humanoid planets in the universe) vary based on the ethics of the era. Ethics generally progress or deteriorate. This determines life expectancy. The humans who lived 80,000 did not necessarily live on planet earth.
See my answer to: How can we correlate Buddhist cosmology with astronomical cosmology? for more colour on this.
In general, one way to try to understand a difficult sutta is to read what Piya Tan says about it -- http://www.themindingcentre.org/dharmafarer/
His translations include footnotes, references to commentary and other authors.
One of the comments from the introduction to this sutta, DN 16:
Steven Collins’ critical remark on the Sutta is helpful here:
[The Cakka,vatti Sīha,nāda Sutta] depicts life in time, however good or bad, as slightly absurd; and therefore its opposite, timeless nirvana, as the only serious thing in the long run. I suggest that the intention (at least in part) of the long-drawn-out sequence of decline and revival, in all its detailed specificity, numerical and otherwise, and also of the humor and irony of the parable, is to induce in its audiences—or at least to make possible as a reaction from some among them—a sense of detachment from, or at least a (briefly) non-involved perspective on the passage of time.
... which isn't a very satisfying explanation, but what do you expect? :-)
My personal understanding of it is presumably unorthodox, but for what it's worth that includes:
The sutta's subject being a "wheel-turning monarch" suggests that's maybe a later sutta e.g. developed for king Ashoka -- I can't imagine why that would have been an interesting topic for "early Buddhists" (like it was argued in this answer that the idea of a "next Buddha" probably didn't arise while the original Buddha was still teaching).
The thesis is that this age (aeon) is worse than before but not as bad as it is going to be. That kind of reminds me of the doctrine that being born human is a good time to practice Buddhism -- maybe better than when you're born in one of the higher or the lower realms.