You can find a lot of ancient people saying it was happening. They were obviously wrong, right? Things are better now than then, so they were wrong. Nevermind that many of them were speaking of real social decline, not of humanity altogether in the long run but of their own society in a less global sense. When Socrates was bitching about "kids these days", Athens was peaking and a long period of uneven decline was starting. So really, he probably wasn't wrong.
Plato, and Heraclitus before him, both thought that society would eventually collapse. They sounded smart but would’ve been quite surprised to see what we’ve done with the world.
Their societies arguably were doomed. I'm sure they'd be impressed by much we have today, but after the shock wore off are you sure they would change their minds concerning humanity's long term fate? Between climate change, the nukes and dark forest aliens, a lot of people today think we're doomed in the long run. (For what its worth, I don't.)
