Many science fiction movies use credits as a type of currency. I'm sure part of the reason is they don't want to assume that one form of fiat currency is going to rule in the future. They also use this because all the signs point to society moving to a world where the type of money won't matter.
Currently, 66% of all in-person sales transactions in the United States happen with credit cards; meaning no physical money is changing hands. I only have cash on me if I'm going to dinner with friends (since it's easier to split the check), or if someone else pays me back for something and they have cash, or when I head to the casino. Otherwise, mostly it's credit cards, Venmo or PayPal. So why does it matter if this is all happening with USD; could it be Bitcoin, Litecoin or even just credits?
The area that is ready to be disrupted by cryptocurrency is the travel industry. Growing up my parents would get things called traveler's checks if we were leaving the United States. Honestly, I don't even think you can get those anymore, and if you did, I doubt anyone would accept them. Credit cards quickly filled that space as a way for smooth transactions and for not having to worry about currency conversion. At first credit card companies were charging fees to exchange this money but we saw new cards come out that didn't cost extra for this action since it took zero work on their end. So most Americans use credit cards when traveling, but there are a lot of people worried about them getting stolen or even places not taking credit cards. The US consumer loves credit cards, but it's not so for everyone in the world. For the merchants, the fees are high, and it might not be worth it to accept them, so you bring extra cash along on a trip just in case.
Anyone who has ever exchanged money from one currency to another before they've left on a trip knows how ridiculous it is how much money the bank charges to covert just $100 to 100 euros. They take a cut in the form of a service charge, just to exchange that money. Costs can add up if you want to trade a lot of cash. Cryptocurrency could change that.
One Bitcoin in the United States is worth one Bitcoin in Japan. That Bitcoin might get you more or less of their Fiat, but what if you never had to deal with that? What if you could just travel and pay with cryptocurrency and never deal with the local currency?
Imagine being about to save up for a trip to Japan and slowly putting money into Litecoin with the plan to use it once you're on vacation. You avoid the fear of your credit card getting stolen, and you don't have to worry about the bank or the currency exchanges charging a fee for the easy task of just giving you your money in another currency.
It's your money, and we're paying for the middleman just to move bits around, seems dumb.
Now, you would need to get merchants on board, but honestly, credit cards took a while to catch on too, we only saw their use rise in the 1980's. So as demand to pay with Bitcoin or Litecoin becomes more desired, the merchants are not going to turn people away.
For mass adoption, we need to see people using these cryptocurrencies for something other than speculative plays. I think travel is ripe for this. As merchants and consumers realize they don't need to pay a credit card company for holding onto their money, we will see a shift.