> Isn't there a third option where the decline is real, proceeds at a faster pace than over several millennia, and leads to the collapse of the civilization of the historical observer?

If this were true, shouldn't we see intermittent periods of "moral uptick" after the collapse of civilization? Either there has been a steady decline (i.e. the next civilization starts lower than the end of the previous one), or the feeling of decline is false due to the missing opposite.