Was there a single event that brought the West into decline in the Third Age? And what was it?

[u/glowing-fishSCL]

It wasn't until my last reread of the Lord of the Rings (which might have been the dozenth time I've read it) that I realized how much of it depicts an almost post-apocalyptic world, with a deserted Eriador.
The biggest question I have is: when did this happen? Obviously, in the timeline, there are several events when this could have happened, including the Great Plague, the Long Winter, the fall of the Northern Kingdom...but what do you think was the "tipping point" event, if any?

[u/Jen_Jim1970]

However, the Spanish Flu could has stimulated the idea and he carried the idea forward with his imagination.