Suppose that the original creators can't (or won't) enforce a license on their software/code, but that work is still popular.

I guess if you want to visualize it, I'll throw out a convoluted hypothetical:
Imagine a very small group of developers that released a code project under an open-source license. The repository was hosted on their servers. However, the everybody on the immediate development team passed away in a tragic accident or something. Their servers shut down after this happened. The project had a fairly large user base, and so others began to host the last revision on their own servers for others to download.

(Yes, I have an active imagination)

Does abiding by the license simply become a matter of morality by its users, or can there still exist a legal penalty when there is no one user or group to enforce it? Could anything be done if an unscrupulous user decided to branch off the project and use it under a different license?

I am not looking for legal advice -- I am simply curious about how software licenses work. I tend to think of strange situations and wonder what would happen in those scenarios.