It's silly talking about "true fans" and "new fans" anyway. MJ has only been gone 15 years, but he's still about to have the biggest biopic of all time.
Versus a Beatles, Elvis, Bob Marley, Freddie Mercury, Bob Dylan biopic, of artists with careers lasting over 60 years time and/or relevance, some of whom have been gone for nearly half a century and belonged almost solely to the elder generation, with far less crossover appeal. You should be glad MJ is beloved by young people, they will help him thrive forever.
If you're upset about the "new gen" now? When this film does gangbusters and it's a whole new wave, buddy, you'll be miserable. Have a great time.