Why can't altcoins erode Bitcoin's foundation? There already are hundreds of altcoins and even more are being created. However, Bitcoin still dwarfs all altcoins in value.

From a technical point of view, Bitcoin was invented 7 years ago and its technology is far from perfect (e.g. forking, speed). Why can't altcoins with improved features beat Bitcoin?
Bitcoin has the advantage of the network effect.
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What if someone creates a new block chain, or a new digital currency that renders Bitcoin obsolete?
Network effect