Never.
I say this because for a few important reasons. Michael lived to the age of 50 yo and he was performing for 45 out of those 50 years. People grew up with Michael. People watched him grow up. He became part of their families. If people could not identify with him in his latter years, they identified with him in his younger years. I believe there was an era for everybody to love Michael Jackson.
Next, Michael's talent was incomparable. We all know that. I don't think anyone can ever match Michael's contribution to music and dance. If you look at the song and dance people we have today. . . who do we have? Chris Brown, Beyonce, Usher, Justin Timberlake, Britney Spears, etc. Sure these performers sell records and are extrememly popular, but have they made the IMPACT that Michael made? Not one comes close, IMO. I mean when you think about it, acts like the Jonas Bros and Miley Cyrus cause a lot of mass hysteria amongst teens/tweens, but are they gonna be around in twenty years with the SAME LEVEL of popularity? Maybe, maybe not.
Michael also always wanted to be the best, make the best music, be the best dancer, etc. His competitive spirit pushed him to do genuis work, innovative work that nobody had ever seen before. Sure, there are always going to be innovators. . . but Michael seemed to always do the right thing at the right time in terms of his career. For example, the birth of Mtv and Michael's Thriller album. His music videos from that album shaped MTV and Michael's album might not have been as big if there was no medium for him to showcase his songs with his amazing videos.
Next, Michael Jackson's appeal crossed the world. He was accepted and loved by people of all races, religions, cultures, sexual preferences, etc. He had universal appeal and his music crossed genres from pop, r&b, rock, hip hop, ballads, etc. He wasn't really isolated in one area of music or dance. He had something to offer to anyone that wanted to be entertained.
Also, just the events of his life seem extraordinary. I believe that people who came in contact with Michael just always knew this kid was special and that he would be destined to do great things. I hear a lot of people who knew him as a kid say that in hindsight.
Lastly, Michael knew how to sell himself.. . well, until it backfired. He was able to create mystery about himself that kept people interested. Sure, he had the talent to back up his success. But he also created this persona, this image that made him larger than life. This attracted people to him, good and bad. He made sure that his name never left people's lips.
Wow. . . this turned out to be longer than I thought. JMO!!!!!