OK, here's really what happened to Millionaire:
First off, the celeberty editions were too commonplace back in 2001. Therefore, the ratings reflected it.
Second, the scheduling went downhill when it went to 4 nights a week. This also showed in the ratings.
BUT, the big reason for its imminent demise in 2001 was the dismantling of the qualifying phone game. It was that aspect that led to the disposal of the original Millionaire in the first place.
Hopefully, ABC will learn from this by keeping the show as civilized(no celeberties), timely(sweeps only events) and more inportantly, effective(a new qualifying phone game).