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 it will never occupy Bitcoin place ya but it can get famouse and success also. The main reason is because Bitcoin is not controlled by anyone who ever mines can use bitcoin or buy and sell bitcoins. But the new coin is created by the developer who is owner of that coin and holds the coin and if wanted can do anything to it with the coin developments.