Monero

Yeah. Two different people had the secret keys for the same wallet. One of them kept them in an air gapped computer. The other person kept them online in a computer accessible via SSH.

Even assuming these two trusted individuals we're not directly involved, having an always online computer with a half a million US dollars on it is a big risk.

I'm in no way trying to second guess the tragedy here. I'm just speaking for people who might have a similar problem on going in the future.

For a shared wallet, something like paperback, using Shamir's secret sharing distributed amongst trusted parties. Could be good. It would require multiple parties to conclude to unlock the key.

https://github.com/cyphar/paperback

The offline wallet signing is really cumbersome, but it is something to use when we're talking about huge amounts of money. https://monerodocs.org/cold-storage/offline-transaction-signing/

I remember reading about air gapped QR wallet signing. https://github.com/nasaWelder/lunlumo which is interesting, but I thought there was something more polished available. Anyway a program that allowed you to easily sign transactions from an air-gapped computer, could be interesting for these trust problems.

So honestly multi-signature transactions are probably the right way to go. It increases the difficulty of hacking the computers to hacking multiple computers