From a copyright perspective, when a work is published, the clock starts on that work. Even if the work is software and subsequently gets modified, if you retain an older copy, it will expire on schedule. You won't get rights to the new stuff, but you could still, say, extract textures and use them elsewhere. Merely endlessly permuting a work does not give you exclusive rights in perpetuity. You keep building rights to the new stuff, but not the unmodified bits.

Of course this point is entirely theoretical at the moment since it will be many many decades before any video game ever created enters the public domain.