You're misinterpreting what people mean by "inefficient" in this context.
The bitcoin mining ASICs are extremely efficient at mining bitcoins. You can't mine them with an old GPU and expect to make back more money than you spent on electricity.
But the choice to mine bitcoins at all is (arguably) an inefficient use of resources that could have been used for pretty much anything else. There's no shortage of useful problems to be worked on by supercomputers and the world would be better off if we did that instead of mining cryptocurrencies. But the economic incentives aren't there.
EDIT: Reading your comment again, I think I was a bit quick on the "you're wrong" here. ISo to clarify, I do agree that inefficiency is at the core of the bitcoin mining ecosystem, but it's all about the "we have to spend a bunch of energy that could've done a lot of other things" inefficiency, not an individual "we can use old hardware for this" sort.
When you look at it from that angle, Bitcoin becomes a very interesting hack on economic incentives (and how they don't always map to useful real-world activity)
The bitcoin mining ASICs are extremely efficient at mining bitcoins. You can't mine them with an old GPU and expect to make back more money than you spent on electricity.
But the choice to mine bitcoins at all is (arguably) an inefficient use of resources that could have been used for pretty much anything else. There's no shortage of useful problems to be worked on by supercomputers and the world would be better off if we did that instead of mining cryptocurrencies. But the economic incentives aren't there.
EDIT: Reading your comment again, I think I was a bit quick on the "you're wrong" here. ISo to clarify, I do agree that inefficiency is at the core of the bitcoin mining ecosystem, but it's all about the "we have to spend a bunch of energy that could've done a lot of other things" inefficiency, not an individual "we can use old hardware for this" sort.