People make mods for tons of games without source code. People are making up excuses and flimsy reasons to get what they want.
This would be relevant if we were discussing US law, but myself and others in this thread are posting replies derived from reasoning beyond Kohlberg's fourth stage of moral development, so please forgive me for saying that the significance of your point here is lost on me.
The preservation of culturally-significant works of art is considerably more important than any definition of ownership found in any law from any country in any time period that you can cite.
> It isn't going anywhere, so even by your own assertion there is no moral right to preserve the source, which isn't needed to enjoy the cultural artifact it creates.
You make an excellent point here, but I'm not convinced that there is no moral obligation to preserve the source code. Granted, it isn't the product, but I don't believe that its preservation value can be easily dismissed given that the product is well-preserved. I will have to think more on this.
Shit: https://en.wikipedia.org/wiki/1937_Fox_vault_fire
Happens: https://en.wikipedia.org/wiki/1967_MGM_vault_fire
If something is culturally significant then it should be preserved and these two hyperlinks should easily make the case that preservation requires distribution.