The defenses people give of file transfers, chat encryption, etc etc are pretty straightforward: certain things should be equally usable by both law-abiders and criminals, for both legitimate and illegitimate ends. That kind of broad protection is the only way to insure that the civil liberties of all users are protected. Some of the illegal things that people do with these tools are unfortunate, but protecting them remains worthwhile.
What I'm saying is that the people who invented cryptocurrency believed basically the same thing about finances - that we would be better off treating the ability to transfer money as an absolute right, kind of like Signal treats encrypted messaging as an absolute right. The fact that the government doesn't see this as "legitimate" is kind of the point. (It was illegal to export many forms of encryption for *years.) As someone who at some points has used cryptocurrency for completely legal money transfers, I've directly benefited from the ease of use and simplicity this entailed.
Is it "naive at best" to hope that Signal will indefinitely maintain the property of protecting my chats against intrusion by third parties? Maybe! But that doesn't change the point that the fact that Signal does this is the whole reason I use Signal!
"I've made the perfect platform for criminals, but for some reason there are a lot of people using it for crimes and now law enforcement is shutting me down! This is an outrage!"
This reminds me of why there are so few ways to simply send someone a large file on the Internet. All of the easy ways make it too easy to spread copyrighted worked, so they all need to suck in one way or another to avoid being shut down.