If the good old days were so good, then why did they lead to the present day?
I have heard two explanations. One is, “they” did it. The other is, entropy did it.
The trouble with “ ‘they’ did it” is that it shines a bad light on ‘us’; for surely ‘we’ were weak, or foolish, or wicked, to be defeated by ‘them’. Weakness, folly and wickedness are not good; so by that account, the good old days were not really that good.
And as for entropy doing it; that makes sense, for entropy increases in all closed systems. Therefore, if entropy is not to accumulate, then your system must not be closed.