I don't believe it would affect USA's financial system. There are certainly a number of individuals who hold BTC who would get wiped out in such an event, but it seems like it'd be relatively isolated.
It looks like some countries where BTC is more popular may take a bigger hit under such a doomsday scenario.
In effect, a 51% attack would cause the attacker to scam a lot of money from people in a short period of time. Then everyone will notice it on the blockchain (remember: the Blockchain is public. 51% attacks would be relatively obvious to see). After that, BTC's confidence will be broken and the value will march towards zero: no point holding a coin when some unknown entity out there can double-spend their coins. That's how it happens for other Proof-of-Work coins.
But at the same time, BTC is the biggest coin, so its the least likely thing to happen. Its far more likely that BTC miners understand the importance of their mission, and remain wasting electricity perpetually.
Some would argue that the BTC community is too vested into Bitcoin. So even if a 51% attack were possible, the people who could do it would never torpedo the coins that the own.
I perhaps downplayed the flippancy in my previous comment. I'm aware of the theory and impact to the cryptocurrency world of a 51% attack.
I honestly believe that the electricity usage - and therefore climate-change impact - of cryptocurrencies to far outweigh the benefits they bring.
I think that the "value... march towards zero" is a good thing because "wasting electricity perpetually" is a bad thing. I don't think we should care about the losses of people who willingly bought into a wild-west scheme.
I don’t know who to root for between the people who don’t care bitcoin is a massive waste in power but engage with it anyhow because I enjoy the hypocrisy and sight of their surface level convictions - or - the people who want tech to crash and people to be burned by that.
I guess the latter as future techs could be made less wasteful.
> So even if a 51% attack were possible, the people who could do it would never torpedo the coins that the own.
So, BTC is popular in large part for offering the ability to evade state controls (both regulation of supply and transfer, and taxation) that fiat is effectively subject to (whether or not they theoretically also apply to BTC), and to avoid control of the banks, but it is unthinkable but that an incumbent nation state, or set of banks, or combination of one or more of the former with or without the latter, would sacrifice the resources to thoroughly discredit BTC?
