That's very frustrating when arguing against blockchain/cryptocurrencies on the internet (and even in places like HN). For every blockchain enthusiast that has a good understanding of the technology and interesting arguments you have a dozen of people who have barely any understanding of how it works and what it can really do and will spew all sorts nonsense, sometimes even self-contradicting themselves in the process. I suppose that some of them argue in bad faith because they're invested in the technology and want to get rich.

This particular issue of the privacy of the blockchain is one of the best examples, I very commonly see discussions in CC circles where it's both argued that blockchain respects its user's privacy and at the same time that we shouldn't be worried about bad actors abusing an anonymous, unregulated system to fuel corruption or highly illegal stuff because "everything is public on the blockchain, it's completely transparent". I guess that's why many of them have this obsession about quantum resistance, there's clearly a state superposition at play in there somewhere.

An other favorite of mine is "what do you mean cryptocurrency is unpractical? I use it to pay everyday using <centrally operated credit card service provided by a centrally operated cryptocurrency exchange using an off-chain ledger that may or may not eventually be settled in bulk on the blockchain>". Sounds like Visa with extra steps to me.