Just because you can't find the copyright holder doesn't mean they don't exist. If your derived work is a big hit, some corporate lawyer somewhere might realized they acquired the rights two generations ago, and bankrupt you with an infringement suit. This is why projects go through the whole clearance-of-rights licensing nightmare, and whenever clearance fails, society loses because those projects never see the light of day.
"If a copyright holder doesn't effectively exist like in this scenario"
I was surmising a scenario where one did not exist to point out an issue with the parent post. Of course this situation would almost never occur in reality.
