It reasonably has to be a gradual decline, actually. Consider: we know from Anamnesis Anyder that not only did some concept crystals survive, but that sundered people could use them. Those things aren't around on the Source anymore, so they ran out at some point.

...and actually. Knowing that they would've had access to concept crystals (and knowing what kinda stuff was in them) gives some interesting context to the fact the First Astral Era was remembered as an 'age of gods'. Maybe those gods weren't the Ascians; maybe it was the creations they still had access to.