Well, to loosely paraphrase Andreas Antonopoulos:
"There are already hundreds of currencies, so what's a few more? There could be multiple successful cryptocurrencies."
I can't see Bitcoin being overtaken by any of the current alt-currencies (maybe in the future...who knows). I agree with Andreas there could specific alts for different services, if an anonymous alt became the de facto currency for the dark-markets it would take a lot of heat off bitcoin.