All this points of yours really needs debate, saying that centralized currency is far preferable than decentralized currency, i think these really need more explanation or insights, it's accepted that centralized currency come into existence before digital currency, which digital can be decentralized currency, because looking at the current situation of nature it's obvious that people uses the digital system of payment which might be cryptocurrency and especially Bitcoin, so it's left to individual to decide for a means of payment, so am disputing the fact that fiat currency is superior than crypto, automatically it depends on the environment we found ourselves for transportation, many people currently accepts crypto because they use it for investment within short period.
So those are my two points- 1) centralized payment systems are faster and cheaper than bitcoin, and 2) centralized payment systems make cash a far better medium for exchange.
Let us face reality and make a positive point towards this, because, if i may as ask, in what function centralized currency is faster or superior than decentralized currency? Secondly you said that fiat is preferable for exchange than decentralized currency.
So i stand to refute the point of exchange, when we say decentralized, automatically we are emphasising on cryptocurrency and directly to Bitcoin, so globally no exchange of any form that can no been done with Bitcoin, except that the environment is adamant or ignorant of digital facility, currently Bitcoin is used for so many exchange's in different countries.