BTC (and block-chain generally) will be used where dollars, Euro's etc. and banking services are not available. Specifically, BTC will "go" and be used where there is no competition.. It will be a slow process, akin to mold slowly growing on the inside walls of a house.. You won't see it for years and then, one day, it is everywhere.
The developed world's speculation on BTC and other currencies was foreseeable but not relevant to the underlying reality: four billion people need access to a reliable currency and a mechanism to exchange it for goods and services in the absence of trusted intermediaries. Modern banks -- a/k/a trusted intermediaries -- are fat and happy with the customers they have. They have no interest in serving this particular market. This is the vacuum that BTC and other currencies will fill.
As BTC and other crypto's show the modern world that such currencies offer a "faster, better, cheaper" alternative to fiat money, then, and only then, will BTC challenge the Dollar, Euro and RMB. My own view is the powers that be will grudging find a place for BTC and crypto's that will minimize the infringement on their economic and political power. The process will be incremental and painful.
Welcome to history. Stay tuned.