Don't be so simple minded. It can succeed as a remittance mechanism without succeeding as a currency. If it does nothing more than eliminate Western Union for international wire transfers it'll be massively successful.
I think by definition it is a currency if it succeeds as a remittance mechanism - because you can really only send BTC using the bitcoin network, it can't track USD values directly (as a network.) Using the Western Union network you can literally "send dollars" (or euros or pounds) but the same isn't true of bitcoin. It's a remittance network tied to a currency, and that currency is bitcoin.
In this sense its use as a remittance network implies its success as a currency.
For this reason, I had no idea this is what you meant - I thought you meant it can succeed as an investment, without succeeding as a currency.
No, IMHO it can fail as a currency and succeed as a remittance mechanism. It can fail as a stable currency and thus not be a valid unit of account while still succeeding for remittance.