When the crypto currencies become so popular and almost everything could be paid in crypto's then I think Fiat would be useless when that time comes.
I think there would be only few people who would still use Fiat .
This may be not so distant in becoming a reality in the future. Cryptocurrencies have risen in user interest lately, and everyone has been paying close attention to their progress in the mainstream world. Banks and governments have been exploring the benefits that Bitcoin's underlying blockchain technology provides, and have considered to make digital currencies of their own. When they start doing this, they might educate people to begin using cashless payments.
Before we know it, people will quickly adapt to buying and selling stuff with digital currencies, and only a small percentage will use physical fiat. One thing for sure is that technology is quickly evolving, and it should only take a matter of time before a cashless society becomes a reality. Nevertheless, one way or another, people would have to adapt to cashless payments as governments use the blockchain technology for their own benefit. Just my opinion