I actually don't see why it's so terrible if I do it offline, air-gapped? Doesn't this directory list ALL potential private keys? It's not as if I'd use page 235 key 15 as my address...my choice would be far more sophisticated than that.
I've been thinking a bit and to me this site could be the perfect cold wallet generator. It gives you a means of choosing an address to store your bitcoin which would be very easy to look up. You remember by page number and location on the page, rather than by address. It means you CHOOSE your address rather than in the case of those other sites which choose one for you at random.
I would like to be able to use this page offline on an air-gapped computer, but the code has been hidden and it's only possible to use online. Is there a way of getting it offline?
Thanks,
Ag
Its an absolutely terrible idea. Humans SHOULD NOT be allowed to choose their own private key from a pool of addresses. The level of security you can expect from an address in directory.io should be considered as insecure as brainwallet generated from key phrase "correct battery horse staple". The problem with you picking an address that is already generated means that anyone else would have a decent chance (higher chance than someone generating a conflicting address) of finding your address and the corresponding private key. Needless to say, the database can be modified or removed at any time. Most clients generates addresses securely and it is never an issue.
Tl;Dr: It is an extremely bad idea and its stupid for anyone to use an address listed there.