The "there can only be one" camp of Bitcoiners do not understand that altcoins supporters don't necessarily think that any altcoin will overtake Bitcoin in value. I can't speak for everyone, but I believe that Bitcoin will always be the "one" cryptocurrency, the crypto reserve currency.* But there will always be room for a few more cryptocurrencies that are adopted widely just like Bitcoin. These cryptocurrencies will have independent economic models and not pegged to Bitcoin and will have a free-floating market price.