Imagine how much energy is wasted transporting physical currency around and compare that to Bitcoin. Doesn't even come close.
Indeed— and though Bitcoin certainly has it's own limitations, it's free of quite a few of the classical ones that plague existing and past currencies— it's thoroughly forgery proof, for example— so the cost of preventing and detecting forgeries and the cost of investigations, trials, and imprisonment of forgers is gone. Bitcoin value can be cheaply protected with cryptography an multi-signature control in a way more robust than any safe or armed guard and while we do depend on energy to secure Bitcoin the energy put in by honest contributors protects everyone's coins rather than protecting just a single vault. ... and not to mention inflation.
But heck even if they don't agree with Bitcoin's advantages, don't see it's applications, and view every flaw as fatal— even if I ignore that they don't offer their own alternative to any of it, and that they seem to miss that all other digital 'cash' like systems have been dead on arrival or rapidly failed— the thing that bothers me isn't that they don't like Bitcoin: there are plenty of reasons not to. ... it's that they repeat the claim that Bitcoin is technically sound after demonstrating conclusively that they haven't the foggiest clue how it works. I find that sad and— considering it comes from people who supposedly celebrate rationality— kinda scary.