You didn't even get into the issues from company acquisitions and license ownership.  The most (in)famous example of this is No One Lives Forever.  I know I'm one of many who would LOVE to see a remaster of it using the latest unreal engine.  But considering that the answer to who owns the copyright of it is buried in a box somewhere in the basement of a building, the only way we'll ever know is if someone basically risks legal actions by rewriting it.. So for all intents and purposes, the game can't be revived.