Well, that was a simple scenario with a simple answer.
But there are many other things powerful adversaries could do. For example, what happens when miner traffic itself is disrupted and the network is forcibly split between China and the rest of the world? Leaving everyone at risk of having their transactions overwritten when the network is allowed to reintegrate?
Anyway, we don't need a 100% effective ban to get an effect. A 90% ban may well be good enough for any practical purpose. Your example is a good one - sure, there are ways to get around 'internet kill switches' several countries have, but in practice these (unfortunately) work.
The fact that some people in a cafe can get one cryptocurrency transfer going is no defeat for a ban, so long as the ban is effective enough to reach its intended effects (e.g. making ransomware useless).
> what happens when miner traffic itself is disrupted and the network is forcibly split between China and the rest of the world?
How would this be done? It only takes a single node capable of connecting to both networks to keep the whole thing working. There are already many nodes working with satellite connections so I'm pretty sure this can't be done even by state actors.
A satellite connection is still dependent on BGP routing to get to China/RoW and still goes via the Great Firewall to connect to China (if outside of China, not sure on how the miners in China do it - possibly they don't have satellite equipment at all). Poisoning BGP routes is well within the capability of state actors, and China could of course decide to block its firewall.
Anyway, we don't need a 100% effective ban to get an effect. A 90% ban may well be good enough for any practical purpose. Your example is a good one - sure, there are ways to get around 'internet kill switches' several countries have, but in practice these (unfortunately) work.
The fact that some people in a cafe can get one cryptocurrency transfer going is no defeat for a ban, so long as the ban is effective enough to reach its intended effects (e.g. making ransomware useless).