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Draw a parallel between crypto and current fiat money systems
Thousands of different cryptocurrencies requiring exchanges between each would create a system similar to what nations have now, ex: exchange rates, fiat currency, etc
In order for crypto to be an effective currency, either worldwide or nationally, crypto as a fiat currency would have to hold its value over time, be universally accepted as payment, and be easy to access and use for all parties. This could possibly mean not being able to easily exchange this currency to a different nation's currency, because the cryptocurrency may be valued as nothing to foreign nations due to the strict regulations.
The elderly or technology-illiterate may find a fully network-based currency difficult to understand and trust, making it difficult to implement and replace the current US dollar. 
Regulation consists of blockchain validations and software rules implemented according to the situation, money transactions. What does this regulation have in mind for things such as transactions consisting of illegal substances, criminal activities, Silk Road-type environments encompassed within the deep web? How would government and law enforcement want to handle these regulations when the entire purpose of a blockchain system is decentralization of a common currency? The blockchain transaction parameters would have to be finely tuned based on what type of transactions were occurring, and potentially monitored for any suspicious activity, once again going against one of the core values of a decentralized system. This also brings up a concern of cybersecurity of how to secure such a large network blockchain against foreign cyberattacks. If this system were to work on any large scale, it has to be resilient to any outside manipulation, or the value of a nation's currency would amount to nothing. (Ledger article 2016 discusses how blockchain technology has the ability to self-govern as a truly "neutral" party, "unable to discriminate between its users", pg. 140)
Any new potential form of currency, especially one with as much media coverage as various cryptocurrencies, would be highly scrutinized by economists as to discover their potential for real-world use cases. (Quartz argues against this, but their idea that early buyers having an advantage over latecomers makes crypto a MLM-type scam does not really apply to such a newly developed technology that 20 years ago, nobody knew about, and therefore could not predict how much of an impact the technology would have in the future.)
Social interactions with crypto technology requires a level of familiarity with how the blockchain system works, even if the current system is not understood fully by some. The system would have to show very significant advantages over the current system to even somewhat convince a voter, citizen, bank account holder, etc., to accept the new system as the norm. 
In movies, specifically sci-fi, we can see futuristic societies with optimized

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