ASIC chip fabrication is centralised in China. The government could seize the means of production, as well as the centralised mining operations that they've attracted by subsidising electricity.
If a handful of governments that control fabs agree Bitcoin has to go, the efficiency of ASIC mining becomes a big threat.
Governments can purchase large runs of ASICs while other mining will revert to GPU. Controlling 51% of the hash rate in this manner isn't as outlandish.
Further, you're not limited to double spend attacks. If you're the government with a decent advantage, you just treat your own chain as true and never accept blocks from other miners. The reward for mining will collapse, since even if you produce a valid block it'll not be on the longest chain once government miners catch up.
And once the reward for mining collapses you can probably even power down some of the ASICs.
There are counter measures, but combined with attacking the financial onramps and making possession criminal, it's hard for me to believe that BTC would survive an attack like this.
But of course, it's predicated on large governments agreeing it's worth seriously attacking. I don't know how likely that is.
Governments can purchase large runs of ASICs while other mining will revert to GPU. Controlling 51% of the hash rate in this manner isn't as outlandish.
Further, you're not limited to double spend attacks. If you're the government with a decent advantage, you just treat your own chain as true and never accept blocks from other miners. The reward for mining will collapse, since even if you produce a valid block it'll not be on the longest chain once government miners catch up.
And once the reward for mining collapses you can probably even power down some of the ASICs.
There are counter measures, but combined with attacking the financial onramps and making possession criminal, it's hard for me to believe that BTC would survive an attack like this.
But of course, it's predicated on large governments agreeing it's worth seriously attacking. I don't know how likely that is.