Number of students seeking biglaw and number qualified and worth a firm competing for are two totally different things. Just because someone at Cooley wants biglaw, doesn't mean they will get it. Good economy or not.
Also, with regards to "when every firm does something it isn't scrutinized as much," while I agree with that statement, it has still yet to be shown that many firms are rescinding offers. There are rumors of it, but nobody has yet to mention a firm doing it by name and it's been nothing more than the usual gossip.
Obviously alot of people who want biglaw aren't qualified, but I think its likely that there are more people out there that could bring something to the table in a big law firm (either now or in the future) than people who get biglaw jobs. If not, I'd still say there is a mass of students out there who really are not all that distinguishable from one another. Some will get biglaw and some will not. I agree with your statement to an extent, but I find it hard to believe that there are exactly as many qualified students out there as open SA positions, unless you define qualified students by the fact that they landed SA positions. Whichever way you slice it, I think demand (even quality demand) will always outpace supply and that tends to give the firms power (guess I did learn something in antitrust).
You are obviously right about no hard proof on the rescinding of offers. I don't really know if/when we would get such proof though. I'd say ATL info is suspect and I really don't see the mainstream media getting interested in the plight of the law student/lawyer anytime soon so I guess we are somewhat stuck with rumor mill at the moment.