So what ways of keeping bitcoins safe do you recommend then? Many people consider hardware wallets as something that is not possible to breach because they were told so. In both Ledger and TREZOR there were discovered vulnerabilities which allowed potential attacker to extract the seed. I haven't heard of any issues with KeepKey. I was thinking of using an air-gapped computer for storing large amount of BTC and a hardware wallet in case I needed to travel and have some bitcoin with me just in case. Have you ever used any hardware wallet?
I think it is good to reduce the attack vectors to a minimum.
Bitcoin Core for example is a software that I trust. It is open source and some of the best developers (that I trust) are revisiting the code. So if you use an encrypted Bitcoin Core wallet with a very strong password for your cold storage, you should feel a lot safer, than with any hardware solution.
Of course, the fact that we have to use closed source computers to run Bitcoin Core, makes it impossible to be 100% safe esp. against state actors.