Laws still exist against things like perjury, even though the existence of the law is not a technical means in itself able to prevent perjury. Note that one of the comments upthread specifically mentioned legislation. The current notion that many people in the tech world have, which roughly states that what determines whether something is kosher is whether it's technically possible to accomplish, is something that needs to change, instead of things just staying a permanent Wild West forever.
There's also an old phrase that putting locks on your doors doesn't actually stop a determined attacker, but that it's okay because they're not meant to—that they're meant to "keep honest people honest". It's a principle that applies here.
No, there are few to no actual privacy improvements over centralized systems.
Perhaps even functional regression: what, are you going to run a hash blocklist across all nodes? Like spamhaus? Is there logging or user accounting? Is anything chain of custody admissable, or are we actually talking about privacy and liberty here?
Is everything just marked, "not for unlimited distribution"? And we dwpend upon there not being bad actors?
Real costs are very different with just friendly early adopters.
Cryptographically signing posts (with LD-Signatures) may help with integrity, but that can be done with centralized systems and does nothing to help with confidentiality.
What about availability?
Is it a trivially-DOS'able system?
There's also an old phrase that putting locks on your doors doesn't actually stop a determined attacker, but that it's okay because they're not meant to—that they're meant to "keep honest people honest". It's a principle that applies here.