Karma isn't a currency, it's formalized reputation to surface good content and gamify writing good content.
> If Wikipedia operated its own currency, it could reward helpful contributions and make vandalism costly
The transaction scale (Wikipedia has two edits oer second) required would put it out of the reach of a crypto though. A centralized Wikipedia-buck would work just fine. Wikipedia itself is centralized.
> (you'd lose 10-50% of your currency if you were found to have vandalized a page, etc.)
This is exactly the type of thing that Crypto was built to avoid!
> micropayments
Due to scalability problems (ie gas fees) aren't cryptos much better suited to macro payments?
> Perhaps HN is wrong to simply hate the idea of decentralized currencies?
You haven't really shown that decentralized currencies are required for a decentralized social network.
You also haven't shown how your preferred networks will avoid the misaligned incentives that have created human misery out of existing social networks if they're still going to be for-profit (and the presence of a coin certainly suggests a profit motive.)