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 ?
i think the altcoin will have huge fans but i am not sure that their fans will use the altcoin and i don't think that the altcoin will be a threat to bitcoin, its too far to follow bitcoin especially in the price. i think people will prefer to use bitcoin and do invest in their money into bitcoin than to make invest on the altcoin which created by famous people.