Yes, the big question is whether currency has such a strong network effect that one will dominate. Hayek argued the opposite, and wrote a book claiming that a system of competing private currencies would be ideal.

There are also a few technical arguments in favor of multiple currencies. At the moment we don't know how to scale a single blockchain to major-currency levels, unless we do mostly off-chain transactions. And with appropriate software, accepting lots of currencies doesn't have to be harder for a merchant than accepting one. There's also some work on cross-chain currency trading without a trusted intermediary.