Well, I guess we devs should also be looking at ourselves, then. A lot of the lax security comes from us collectively choosing to build applications using cloud services that talk to each other over the public internet. That pretty much describes the so-called "modern data stack."

How would such damages be assessed or proven?

They would be assessed according to rules written by people who are skilled at writing such rules. The rules would be evaluated by looking at data over time and revised as needed by experts in the industry who are as neutral as possible, maybe with some feedback from the public. The courts exist for any contention regarding responsibility.