> For example, I go to buy a t-shirt for $10 and I pay the merchant $10 equivalent in bitcoin cash.
This is why I don't think of bitcoin as a currency: I've never seen it used to denominate prices. As far as I can tell, not even Silk Road had prices set in bitcoin.
Paying "$10 equivalent in bitcoin cash" (or bitcoin, or litecoin, or whatever) is using those cryptocurrencies as a payment gateway; the currency in those transactions is $.