People could vote in a decentralized way, and the voting could be based on Blockchain. Even at this point, it would be Blockchain doing the replacement, not cryptos. But then someone has to decide what to put on vote, someone has to manage institutions and ensure communications with other countries, someone has to write the laws to vote on etc. I don't think there's a way to make all of it work in a decentralized way. Also, the majority is very ignorant on many questions, and it would vote toward a catastrophy. If a question of abolishing taxes was voted on, the majority would surely support the decision. And then there would be no money to fix the roads, subsidize medicine and public transport etc.
You make a lot of interesting points. I threw this idea out there to crowdsource others' ideas as well (decentralized!). FWIW, historically pure democracies have always failed in the end.
Someone would manage institutions. Here is a general idea on how it could possibly work in a decentralized way:
The structure would be set forth in something of a Constitution when the initial cryptocurrency was created. Parts of it would probably need to be immutable (unchangeable).
The leaders of institutions like defense would be voted on through the blockchain.
Salaries would be paid for through smart contracts.
As for abolishing taxes, mob rule is a consideration.
"Taxes" would be established by a fee for using the cryptocurrency.
The "budget" would be established by setting percentages for different resources. i.e. defense would be 50% of the total fee, safety would be 25%.
If people wanted to change those allocations, they would have to take from one of the other resources.
The problem with many current systems of government are that the mob encourages politicians to keep adding services and government responds by raising taxes (%) or printing more money and devaluing the currency.
Instead of cutting one resource to add to another, everything is obfuscated and the overall budget becomes 200% more than the taxes that are collected.
This leads to the eventual downfall of that government.
With a budget that is published on the blockchain and the overall fee is immutable, people would see the impact of taking money away from one resource to add it to another.
If roads start to deteriorate, funds could be taken from defense. If medicine needs an increase, funds could be temporarily shifted as well.
The budget could never go over 100%.
Government expenditures would be transparent on a public blockchain. No more $1 million for a hammer, $10,000 for a screw. No more bribes.
Best(?) of all, it could all happen instantaneously, instead of waiting for years to elect new officials while things deteriorate under elected governments.
This form of governance would work best on smaller countries. Having different rules or different side chains for different countries would likely make sense.