Several things wrong with this question/poll.
Firstly, it's not a downfall year -- the downfall has been going on for several years now.
Secondly, it's far too simplistic to try and blame this all on one person or thing. Way too many factors. Way too many mistakes. And way too much denial.
However, if I were going to try to pinpoint the exact moment that it started... I'd go with May 8, 2003 (as mentioned elsewhere), the day Chris got injured against the Mavs. Team's never been the same since. And not entirely because we were without our best player for so long. A lot of it, I think, has to do with how well the team played to start the next season while Chris watched. I'm of the opinion that a lot of the asinine "moves" and patch jobs that we've seen since have been a direct result of that incomplete team fooling the organization into thinking that you can be successful without either a superstar or a dominant big. Utterly wrong on both counts. And stupid.
Part of me wishes Webber had never come back that year, if only to prove, once and for all, that Peja would not have been named MVP and that the team would not have gone on to win a championship without Chris. At least then the powers that be (and I mean ALL of them) might have gotten this pretty-but-irrelevant silliness out of their system.