[coldtea]

Another reason is many things always going downhill, and people remembering them, while younger people grew up already familiar with the decline.

That's not very different a story than the continuous progress myth, just has an opposite sign.

Fact is, some eras progress for the better, and others regress (Weimar Republic, the fall of Rome, and countless other examples). And that's overall. In specific things, it can vary as well in opposite direction to the overall trend.

And long periods are relatively stagnant, such that people for centuries on end, or even millennia, live more or less the same (e.g. most rural areas in Europe between 400 A.D and 1900 A.D.), and few speak about the "good old days" there.