I'm brainstorming ideas for a fantasy videogame and have been trying to list possible reasons why a fantasy world would have a lifespan diminish exponentially through countless cycles of death and rebirth, each dominated by a new sapient race for the duration of the epoch until they face their predetermined apocalypse. Each successive peak just falls short of the last, and each dip the inverse.
Humans who live in the current Nth epoch are already facing the decline. As far as their descent, from the golden to silver to bronze to the current age the decay has been taking place faster.
The splendor of the earliest age, who is without compare, will never be repeated. When it all ends the apocalypse comes as if to wipe the slate clean.
This world was created by gods who in turn may suffer their own (considerably longer) decline in their powers and lives. They, as the humans, may not know of this decline. either way they appear to never intervene.
For civilizations think H.P. Lovecraft's at the mountains of madness, where the elder things' decline takes place over vast eons to end finally with their last legacy being the mindless Shoggoths or Tolkein's The Simarillion, where after the time of the elves domination over middle earth passes and mankind comes to take their place unable to repeat truly their splendor, gradually declining themselves after Numenor.
I may not quite grasp the concept, but for Hindus there is the concept of Kali Yuga, for Buddhists the time of the Maitreya shall be a time where chaos and vanity rule and Buddhism is forgotten, and the Greeks identified their golden age with the now ended reign of Kronos, whose son Zeus now presides over heaven.
The world and civilization before it dies and is reborn every age, which each get successively shorter.
What are possible reasons a world's cyclical lifespan would get progressively shorter after each cycle's apocalypse?