I don't think it will be that few, just because there are use cases beyond "currency." But in terms of currency, yes there is limited room for success. Part of what I like about crypto is that it is so much simpler than having to deal with USD, EUR, JPY, GBP, etc. But if you got 50 different spending currencies, then I guess that's not really the case. Probably will be a mainstream currency and a privacy currency that will operate side-by-side and become adopted as a side option to existing centralized solutions.