You know, I experienced the same thing - a very quick progression from functional to almost on the skids...starting to miss a lot more work where people were actually complaining (it wasn't just me complaining about it!), driving drunk too often...just going downhill fast. Seems to me once they go past that "functional" stage, the slide just gets faster to bottom.
I also watched my friend slide downhill quickly, but that functional stage lasted a long time - almost a decade for her.