Cultural pessimism arises with the conviction that the culture of a nation, a civilization, or humanity itself is in a process of irreversible decline. Wikipedia

An attitude that sees sociocultural changes (in general or in the present situation) as predominantly tending towards a less moral, less constructive, and/or less preferable state of affairs; particularly, the idea that at some point civilisations and cultures reach a peak after which they must inevitably decline. Wiktionary