The full circle was part of 3rd ed. The fall of Mankind like civilizations rise and fall. So yes, once it was a possibility.
Now we got the reduced "end times" theme, which contains a lot of "time" lacks an "end" and is generally seen as the Darkest hour.
After a 'darkest hour', there may be brighter ones...
But mankind can fall like every species. The Galaxy is filled with ruins of ancient lost civs.....
Is there a chance it happens?
Not really as the background is tied to the main actor of it, and thats humans, be they imperial or minions of the ruinous powers.