I think the main culprit for boxing's decline in the US is your promoters (and this is a problem globally but this threads about US).
They seem to be trying to screw fighters for money constantly at any opportunity, they dont look after fighters after injury/retirement (Mcclellan).
Putting all big fights on PPV and putting some moderately big fights on PPV means casual fans only ever get to lower level fights.
Theres no investment back into local communities boxing programs, look at say football (soccer) in England, every club spends millions scouting and training talent, as does the FA. This doesnt happen in boxing so its left to dedicated trainers mostly from poor backgrounds to start up there own gyms at there own financial risk.
The growth of NFL/NBA is only a PART of the decline of boxing, you have to look at why those sports continued to grow while boxing has stagnated. For these sports its about investment in the future of the franchise, for boxing its about making quick bucks and getting out. Ive heard people in this thread saying how the growth of HDTV has made NFL more popular, but why is that any different to boxing? is it because for boxing you need to fork out for HDTV, subscriptions to like a million channels AND then PPV for big fights?
Having no central boxing commission doesnt help either as there is no coordination throughout the boxing world like there is in football with FIFA.
Either way boxing seems to be doing fine globally but like others have said earlier (and it pains me to admit this) America is the Mecca of boxing, when its good there its good everywhere.
Plus id love to see Wlad and Vitali Klitschko dominating all these 'super athletes' you have in the ring