Bitcoin will never be the sole currency just like the internet is not the sole source for news, phones are not the sole way to communicate etc...
How about cryptocurrencies in general? Anything that requires some kind of prior knowledge, 'luck', pre-existing wealth, technological capability etc. will eventually run into the same problem.
What kind of problem? It's not a problem if it doesn't become the sole currency. I don't think anyone hoped for something like that anyway.