I beat you by one post!
I was there; not on the bridge, but about a mile away to stay away form the crowds.
I would love to hear from an engineer that knows,but I think the situation that day was very serious.
The full article I posted from the Merc quotes several engineers who say there was plenty of capacity left in the bridge and collapse was nowhere imminent. There was a much bigger risk of widespread panic and everybody trying to get off the bridge all at once.
If you take 300k people and assume an average of 150 lbs, you're talking an added 45 million lbs placed on the bridge that day. The main span of the bridge itself is 4200 ft long, which results in a load of approximately 10715 lb per linear foot. With a width of 90 ft, the load resolves to approximately 119 lbs per square foot.
YMMV, these were quick calcs I just did while drinking coffee.