Some douchebag will leave him off. Griffey had 100 percent of all the public ballots as well, but three folks ended up not voting for him.
according to the article one guy said he won't vote for Rivera as he doesn't think highly of saves as a stat, but he's not sending in his ballot so as not to potentially screw up Mo getting 100%. But you are right, someone won't vote for him. No way he gets 100% when Griffey, Seaver, Ryan, etc., didn't get it.