Could state authorities shut down Bitcoin at any time? A state-sponsored 51% attack would be very difficult. Even though a lot of the mining is in China, it’s not as concentrated as you think; some of the pools are, but the mining farms themselves are distributed in very remote rural areas. In those areas, the mining farms represent a significant percentage of the local economy.
Before a hired gun gets up and tells them to shut down the mine, you first have to pay them a salary, and the Chinese government is a minority bidder because the mining farm operator is paying them much more lavishly.
Don’t underestimate how much resistance you’re going to see in areas where this is having a significant impact on incomes in rural areas that have excess capacity electricity. The biggest threat for the Chinese government isn’t Bitcoin or America, it’s a billion peasants with pitchforks who have decided they’ve had enough. They’ve been trying to forestall that from happening for 25 years.
Economic growth in rural areas is the goose that lays golden eggs, which benefits them too. Don’t overestimate how effective a 51% attack would be either, or how long it could be sustained, and what kind of countermeasures we could put in place. This is a system that will evolve, not just sit still and take the blows. After 7 years of Bitcoin, it’s not because they’re not trying or that they don’t want to stop it.