I don't see NXT replacing BTC. I could see a future in which a couple cryptos co-exist. Visa, Mastercard, American Express.
I think there will be room for hundreds of altcoins in the future, each serving its own niche function. The reason why there are only 4 or 5 major credit card companies is because anything close to a competitor gets bought or legislated out of existence, similar to how Ebay and Amazon deal with other online merchants.
Since cryptos are currencies and not companies they are not responsible to shareholders, they don't necessarily have motivation to swallow each other whole or knock each other out of business.
For sure, hundreds will always exist, but I doubt more than a handful will reach a useful mainstream status and mean anything to the world economy.