In case someone famous created an altcoin, but not someone famous like Linus Torvald or whatever obscure cryptographist, no someone that is mainstream, like Rihanna, would it be a threat to Bitcoin ? You could be sure that masses of people would come to use Rihanna's payment system, as it would be branded. Or at the inverse would that be the door for millions of people to know what Bitcoin is and make them understand that this is a great progress ?
For me I dont see that as a factor in determining whether the world will accept it or not. Because I have seen successful musicians sucks at any other thing other than show biz. We have seen trump coin here and I didnt even bother to check how its doing so far its not taking over from Monero for it to come close to bitcoin. Some business are just meant for specific set of people and crypto to me is not meant for entertainers.