your answer is very clear short and concise, the worst case scenario for bitcoin is the price of bitcoin falling to zero, if the price falls to zero, maybe most of us will be severely depressed, especially the whales who hoard a lot of bitcoin, they will definitely hang themselves, but in my estimation it is will not happen because considering bitcoin is now very popular, even the government has adopted bitcoin..
This is so far the worst case scenario for all of us if cryptocurrency token lost it's value but I bet that this kind of situation is far from reality. I don't think that the country's around the world could come up in building one blockchain but the only possible thing is that they every nation could make their own kind of crypto which will only adds up to the number of altcoins. But over all it will be hard for the government agencies to interrupt with the blockchain of bitcoin and ethereum since it is a decentralized finance.