Well you're asking a rather loaded question without much possibility to definitively answer it. In terms of multiculturalism vs nationalism, I think most people are inclined to have their sport loyalties gravitate towards the local, or the country athlete. However post the 1970's or so, at least in America, there began to be an exploration into other cultures and less of a steadfast support for whomever the athlete is that lives in the same borders. Especially in combat sports. Now that doesn't mean America still doesn't enjoy a roaring "USA! USA!" chant, but as many of the top athletes in the world began gravitating towards our shores for the purposes of training at the best locations with the top trainers, and they began to fight more and more in our cities, they become almost adopted on a national level.
Manny hasn't had a single fight in the Philippines since 2006. Meanwhile the bulk of his money has been earned in fights in Vegas, he trains with Roach out in LA, and he's become a fan favorite to many Americans because they appreciate his style, or his personality, or whatever it is they find agreeable about him. On the reverse side of this coin, we have Floyd, an American born fighter who has openly talked about cultivating a persona to hype and sell tickets and buys with the knowledge people want to see him lose the 0. To barrow from pro wrestling lingo, Mayweather is a prototypical "heel" - the guy who's on top of the world, has the money, the record, the fame, and the skills to back it up and isn't afraid to be "hated" because he knows that translates to more ticket sales. All the flashy things he does are to garner attention, do you really think when cameras aren't rolling he tosses hundred dollar bills around like they are confetti? No, it's an act. So we have an instance where America has grown to love one fighter who was born out of the country, facing another fighter who they dislike who was born within the same country.
Someone earlier brought up the Yankees analogy with Money, and it's quite fitting. I know born and bred New Yorkers who HATE the Yankees, and are either Mets fans, or Phillies fans, or take your pick really...so come World Series (not recently, obviously) just because the Yankees are in the finals doesn't mean they are suddenly going to abandon their years of built up animosity just because the locals are involved. Same concept at play here.
So I don't think this is a sign that nationalism or patriotism is on the decline, merely that the narrative of these two athletes have been established for years now, and neither one particularly plays up their nation of birth in terms of why this fight is significant. Obviously Pac is going to carry a sweeping sense of support from the Philippines as the country has rallied behind him both in terms of boxing, politics, and even letting him and his laughably poor basketball skills be excused in exchange for being the National Hero. Mayweather seems far more content to be enveloped in celebrity culture, which works for some fans and sours others.
As for this fight holding American Boxing superiority in the balance? Ehhhh, I'd argue we lost the image of being the boxing power house quite a while ago. I've long said, it is going to take a dominate American Heavyweight Champion again to actually reignite the level of attention and fervor it would require for a country to become a super power in the sports world.
Very good points you brought up. As sad as it is to say, it will take an American heavyweight to reignite public interest. All that further goes to prove is that the people have yet to realize true skill over brute strength and size. Manny Pacquiao and Floyd Mayweather are truly gifted athletes which the public should appreciate, regardless of alliances.