Because of network effects, there will likely be only one dominant coin. Whether or not other coins disappear completely or limp along as someone's hobby doesn't matter much. This doesn't require universal agreement. I'm sure lots of people today disagree that facebook is better than myspace -- doesn't matter, facebook still won.
Early Bitcoin adopters, when they see that nucoin1 or nucoin2 will likely be the dominant coin, will back nucoin2 because they are heavily invested in the nucoin2 blockchain. The incentive comes from knowing that Bitcoin will be replaced, and realizing that they are much better off if it gets replaced by nucoin2 rather than nucoin1.
The creators and early investors in new coins haven't duplicated their new coin's blockchains because with their current block chain they are the early adopters and they stand to gain from pumping it. They know their target market for their new coin consists mostly of people who missed out on Bitcoin's early days, and are hoping to get rich off of an altcoin. These coin creators see that they don't need to create a coin that threatens Bitcoin to take advantage of their target market. It's easier to make money with an altcoin by starting a new chain and convincing other people that if they invest early in the new coin they'll be rich.
The idea is that early investors in Bitcoin will be the ones to create a fork of RandomAltCoin, if they think RandomAltCoin has a legitimate shot of taking over. The reason this hasn't happened is that no altcoin currently offers any significant innovation over Bitcoin. Bitcoin early adopters know that they have nothing to worry about.
Sunny King seems to believe that peercoin is a significant improvement over bitcoin. The genesis block for peercoin is less than 5 years younger than the genesis block for bitcoin, and he could have simply migrated the bitcoin blockchain rather than starting a new one from scratch. All adopters of bitcoin would have been included. However, he doesn't have an incentive to do that, and he acted rationally by starting from scratch, given his belief, because he and other early adopters of peercoin stood to gain more by starting a new blockchain. That is true regardless of whether Sunny King himself was an early adopter of bitcoin. He can still have his bitcoins, and get richer off peercoins. The same is true for other altcoins, regardless for whether they are pump-and-dump scams. Some altcoin developers might or might not have been early adopters of bitcoin, but that point isn't even relevent, because it doesn't change their incentives. If some altcoin replaces bitcoin as the dominant player, the process will be slow, giving early adopters of bitcoin plenty of time to jump ship and stay rich.