The Klitschos average about 50,000 people at their fights, plus the worldwide tv audience. ESPN had a hissy fit with them a few years ago and refused to show their matches. Wladimir makes more money in one fight than the MMA makes in a year,that is not a sport in decline.
These days, a championship bout barely gets mentioned and the average guy on the street not only cannot tell you who the current heavyweight champ is but they probably couldn't even name one besides maybe Muhammed Ali and the guy who sells grills on TV.