What OP suggested is similar to Euro, a single currency across boarder, and you see how much trouble it has now 
US can not force Germany to use USD as currency, because printing currency is the biggest robbery power central banks hold, they will never give up that right to anyone else, so what OP suggested would never happen in the form of a decentralized system, since a decentralized system do not allow centralized control of money supply.
From consumer point of view, the closest result is like VISA or paypal, you do the transaction in a third party payment processor network, and they do the real settlement afterwards
Blockchain technology itself worth nothing, because once invented, you can duplicate it thousands of times. Anything with unlimited supply will definitely worth nothing
But bitcoin is limited, and the cost to mine coin and maintain the network is very high, that indicated the competition to get bitcoin is also very high. Without that huge amount of infrastructure investment, the bitcoin blockchain is as useless as thousands of other altcoins, just some lines of code in github, a freeware
And I have never understand that smart contract thing, how could you ensure the validity of the promised assets in those contracts? These contracts have no legal validity and the counter party can just run away or claim to be hacked as we have seen many times in this space. The only thing blockchain can ensure is the ownership of bitcoins, if bitcoin worth nothing, then those ownership will have no meaning
So the most important thing is bitcoin's blockchain, you can not separate these 2 concepts
US can not force Germany to use USD as currency, because printing currency is the biggest robbery power central banks hold, they will never give up that right to anyone else, so what OP suggested would never happen in the form of a decentralized system, since a decentralized system do not allow centralized control of money supply.
From consumer point of view, the closest result is like VISA or paypal, you do the transaction in a third party payment processor network, and they do the real settlement afterwards
Blockchain technology itself worth nothing, because once invented, you can duplicate it thousands of times. Anything with unlimited supply will definitely worth nothing
But bitcoin is limited, and the cost to mine coin and maintain the network is very high, that indicated the competition to get bitcoin is also very high. Without that huge amount of infrastructure investment, the bitcoin blockchain is as useless as thousands of other altcoins, just some lines of code in github, a freeware
And I have never understand that smart contract thing, how could you ensure the validity of the promised assets in those contracts? These contracts have no legal validity and the counter party can just run away or claim to be hacked as we have seen many times in this space. The only thing blockchain can ensure is the ownership of bitcoins, if bitcoin worth nothing, then those ownership will have no meaning
So the most important thing is bitcoin's blockchain, you can not separate these 2 concepts