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.
1. Morality of a society is not a single dimension. There are multiple aspects of life in which society could become more or less moral, depending on what activists choose to fight for (e.g. exploiting other's labor)
2. The morality level doesn't have to go straight up, down, or be flat. It can move like a wave (or in some cases, a roller coaster), increasing or decreasing over different eras as people learn and forget the value they derived from certain morals.
Because of confirmation bias. When doomsday prophets are wrong, they're rarely remembered. When doomsday prophets are right, was it because they had insight that others lacked or was it simple chance?