A world without fiat currency? Hmm. I wouldn't say that would be good. Cryptocurrency needs something fiat doesn't. Internet. I can mention many examples of why fiat is better than bitcoin on some important spots of life. This over enthusiasm of this community has lead to the belief that fiat does
always bad. Inflation is what have made it bad, otherwise it's pretty useful. You can transact without internet immediately.
I do believe that bitcoin brought us some new ideologies about money, like that it is not minted by a single authority, it is anonymous, it's not physical etc, which really helped me, personally, to understand money better.
We do forget something very important. Fees. If the whole world started using bitcoin instead of their national currency, bitcoin would be... useless... Who would pay a thousand dollars in fees? I have mentioned it several times on this forum. I consider bitcoin as a store of value, not as a medium of exchange and I do that, because it looks more like gold than dollar. Both gold and bitcoin have value because people think they should. On fiat government forced everyone to believe it has value. The fun fact is that everyone is blaming government because dollar is losing it's value. Guess what, no one told you to keep it. You can buy the analogous assets to store your value, but not on fiat. Dollar is a medium of exchange, gold is a store of value. Do you understand now the difference between fiat and bitcoin? This is how I see it.
Other cryptocurrencies, though, that have less demand and you can confirm transactions faster and cheaper, can be used for purchasing stuff. (e.g Litecoin)
The halving does not reduce the number of bitcoins. Bitcoins are not consumed. The number of bitcoins increases until it reaches 21 million. It doesn't go down.
The supply doesn't decrease. Although, the available bitcoins do and since they do it makes the currency even more valuable. (
Article)
Ethereum is not centralized.
What needs cannot be served by Bitcoin that can be served by fiat currencies?
I can't buy a coffee with bitcoin. And even if I can, I won't pay it $2, but more than double. If the coffee shop wants at least 1 confirmation then I'll also have to wait some minutes next to the cash register.