Uh, my paper dollar is backed by the might of the US government and the US economy. It's a lot more than just "shared acceptance". Countries couldn't just switch off the US dollar reserve currency for international trade without massive economic repercussions, which is why no country or set of countries has developed a viable alternative to the dollar for a global reserve currency. It's not like China/Russia wouldn't like to, they have no choice.
The EU came the closest, but no one outside of Europe has wanted Euros since the Greek economic crisis. Hell a large chunk of internal European trade is still done in dollars on the back-end.
On the stick side, under Obama the US stepped up the various mechanisms such that we can cut off trade denominated in dollars on a whim to anyone we want, that's part of how sanctions work. If we (the US) decides we don't want a country to be able to trade internationally, we just cut them off from converting into dollars at the exchanges. There are no economically viable ways around this at present. Just ask Iran.
Bitcoin however, will collapse the moment enough of the crowd loses the faith. It'll go on for a while because it's the new hotness, and some people will get lucky and rich, but at the end of the day it's an ultimately worthless commodity (unless you're trying to avoid government regulation over financial transfers) that exists as long as world governments allow it to exist.
Sure, if theoretically some critical mass of the population all collectively decided Bitcoin was the future it would be, but right now we can't get a critical mass of US citizens to wear masks during a pandemic that's killed hundreds of thousands. So I wouldn't hold out too much hope for pan-Bitcoinism.