What did they do? They ignored common sense.
The bitcoin dev responsible for this idiocy is totally incompetent and should step down effective immediately. Oh wait, that happened.
Carry on.
It's easy to cry "I told you so" in retrospect. But there could have been an exploit in any of the other dependencies. Or in the Bitcoin P2P or RPC network code itself. By no means is OpenSSL the only software that has bugs.
The only long-term sustainable solution to key theft would be to isolate the private keys and signing from the wallet in either a separate process, a trusted computing module or even a seperate device (in order of increased security).