The two year headstart is also a bit of a misnomer. Growth was rather stagnant for the majority of that time, with a high rate of growth over more recent periods. Really Bitcoins major advantage is being first to market and having a small geek community base. First to market doesn't matter if the model can be copied and the Bitcoin community is nominal in scale. I'd be curious to hear of any serious barriers to entry by a competing crypto-currency because I don't see any.
But I don't think that the model can be copied so easily as you might think. Sure, one can take the code and compete, but then you are starting from a community much smaller than even Bitcoin. As the system presently exists, security and value come from the network effects. So any particular merchant or consumer is going to favor the established blockchain, even when he doesn't really know why this is in his own interests, because some will understand this, and the ignorant user will use whatever he has greatest exposure to. This will always be bitcoin unless and until a competitor with a distinct advantage can be developed, and such an advantage cannot be one that bitcoin itself cannot reasonablely assimulate. In the long run, I think that we will see both specialized parrallel solutions such as namecoin, and regional solutions. Yet I think that they will all be tied back into the main bitcoin blockchain in some fashion that allows the other smaller chains to 'piggy back' on the security model of the main blockchain. But then this kind of co-dependency also solidifies Bitcoin's main chain as the common thread, and thus bitcoin as the currency of international Internet trade.