It's not hugely different to if you had $150m in Albanian lek or Abkhazian apsar. You would find it ridiculously hard to convert into an alternative currency in one go and even if you did you'd probably be getting a bad deal (and could even devalue the currency in those two examples).

Ultimately, there's always a price that you can convert ("cash out") anything at. For example, I'd be willing to pay $100 for 100 bitcoins right now. Eventually a seller and buyer will find a meeting ground, a transaction is made, and value is defined.