I have been thinking a lot about RUIN lately and the reasons for why some things are true. The biggest thing that has been bugging my mind is how are Freddy and Monty still active? All of the animatronics are messed up, but the two are special cases.
Yes- they are both extremely wrecked, however, I am focused on 2 major issues: Freddy, his head, and Monty, his legs. Or, well, their lack-there-of. Both of them are missing key components to operate.
Talking about Freddy first, in Security Breach we see in both the Princess Quest Ending as well as immediately before to the initial Parts 'n' Service repair minigame that he is fully functional as just a head.
This tells us that his AI, battery [as seen in the Van Ending], voice box and all other essential parts of him are located there. Because of this, we can make the assumption that Prototype Freddy has none of these. Without his head, Freddy should be completely out of order.
Moving onto Monty, we see in his Decommissioning Scene that he gets broken in half, losing his legs. Since he is still able to move around at first glance, he seems more-or-less functionable.
However, we know that the animatronics charge via the Charging Stations located around the Pizzaplex. From what we see, it appears they charge through their soles.
Without his legs, it is quite literally almost impossible for him to charge. Freddy can at least charge through his ears, but Monty has no appendages showing from his head. His hair is not metal [like Roxy's], and unfortunately, it can be assumed it is nonconductive. Without a charging source, he will eventually lose power and stop working.
From looking at the damage, abandoned construction and graffiti, my guess is that RUIN comes out at least one year after Security Breach. By this time, there is no doubt that the animatronics have needed to charge. Since there is running water, it can be assumed that there is also electricity.
So, since it is completely improbable that Freddy and Monty could still be active, why are they? My conclusion is that the Virus is stored in their mechanisms, not their internal systems. Freddy states in the True Ending of Security Breach that he is able to hear his friends in turmoil, meaning their AIs are not affected by the Virus.
This is why I believe Monty is so rabid in RUIN. His AI is most likely hibernating from lack of charge, so right now his body only has the Virus moving him around. This is why he only growls and never speaks, as well as why he has no self-preservation and will slam into walls left and right- he has no AI active to articulate sentences or to alert him to things outside of his directive.
