They would need to prove that they have the rights to it to win court, yes. The chances of Activision actually pulling the trigger and suing if you made a remastered version are definitely less than 50-50. They'd have to actually have owned the rights in the first place, they still have the documentation to prove it, and that they'd find a suit a profitable idea? Let's say it's a 1/3 chance. That means if you publish it, you'd have a 2/3 chance of not getting sued by Activision, that can have a positive E(V) if you just go ahead and YOLO it.

But the killer is that WB and Fox (now Disney) also are sitting out there as maybe rights-holders. Let's say that each of them also has a 1/3 chance of suing and that they are all independent. Now you have a 8/27th chance of not getting sued- less than 1/3. So the expected value has to be twice as large as with a normal, single company situation to justify the increased risk of lawsuit from one of three companies. And so no one pencils out the choice as a good one, compared to the opportunity cost of working on some other game with a clearer rights situation.