I don't know how effective you think the great firewall is, but it's definitely not effective enough to "trivially stop" 500 bytes of banned data from being transferred.
For a repressive government, that means that most people will steer clear which makes it easier to filter since you don't have a large volume of innocent traffic complicating analysis and by creating a great mechanism to get people to install spyware by circulating news underground of a great client which is preconfigured to evade the filter.
If this is your threat model, cash is much safer.
you only need to be "on" to make transactions. In the case of sending transactions, you only need to broadcast the transaction itself (500 bytes), with no further traffic needed. The blockchain data is also broadcast via a satellite: https://www.blockstream.com/satellite/
>transferring non-trivial amounts of data
SPV clients require very little traffic, on the order of tens of kilobytes. Full nodes only need to download under 2MB worth every 10 minutes, or 3.33 KB/s. That's very easy to sneak under the radar if need be.
>Do you really think it's that hard to block port 8333 and a list of known hosts
good thing you can run bitcoin as a tor hidden service.
>If this is your threat model, cash is much safer.
Except you need to be in-person to make transactions.
This is a common sales pitch but it's relying on the listener not thinking about the problem. Any government can prevent you from using Bitcoin by blocking your transactions: every transaction requires a network exchange and the same infrastructure which is used for things like the Great Firewall can trivially stop that as well.
More importantly, however, that's a hypothetical situation for most people here. The realistic threats to think about stem from the difference between you being able to _make_ a transaction and not have any consequences for doing so. Bitcoin requires you to publish a full log of your transactions for your government to see — if you don't report transactions, skip paying taxes, or conduct a transfer with someone on a blacklist, they can use the public ledger as a signed confession.
Every transaction you make can be used to deanonymize you, and that sharply limits the use of Bitcoin because there's no demand for random numbers — only real things you can do with them, which require physical presences which governments control. If your goal is to transfer money and flee the country, never coming back and writing off any friends or family left behind, Bitcoin might help. If your usage is anything else, you're just counting down until you or someone you interact with has their data compromised.
Now, maybe you've also heard the marketing claims about mixers. Think about that a bit more carefully and you'll also realize the problems which make those unsafe in practice: beyond the expense, you're trusting the mixer operator not to be compromised, simply having any transaction linked to one is evidence that you're doing something illegal, and you have to worry about other users of the service using it for crimes which are serious enough to get more investigation than you personally are worth and proving that you were not knowingly helping those people.
If you were running the secret police somewhere, one of the best things you could do would be set up Bitcoin exchange and mixer services and promote them in your country so you'd have people giving you signed evidence of things you probably weren't even aware of.