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An unpopular note, to say the least, but certainly a significant one: if Marquez beats Pacquiao, it could give many fighters a good reason to retire.
For Marquez: There's no better way for a 38-year-old fighter to retire than on top of the world, and Marquez is smart enough that he may decide this is a perfect time to call it quits. Then again, the list of fighters who retired at the right time is far shorter than the ones who held on too long.
For Pacquiao: If he sees a Mayweather bout as an impossibility now that he is not the undisputed king of the sport, then there isn't all that much left for him to do other than retire and go back to serving his countrymen as a political leader. It's hard to imagine him not taking a Marquez rematch, though, if the option is available.
For Mayweather: The guy loves announcing his retirements, and since he's so concerned about keeping that "0" in the loss total, this would be a perfect chance for him to exit. He could say that he destroyed the guy who beat Pacquiao, and so Pacquiao wasn't a legacy-defining guy for him to fight anymore. We might not believe it, but that's never stopped Mayweather from saying far worse things in the past.