The problem is they both make sense.
John lacks any agency, and as his abilities start "failing" he loses more of what little he has. The only things he can do is perceive the world through Arthur's eyes and talk to him in his head. That's it. He has no material effect on the world around them. He is solely 100% reliant on Arthur for his continued existence and influence on the world. He can't ignore Arthur whatsoever (unlike Arthur who can ignore John easy peasy) he can't Do anything but talk and observe (and also experience the deaths of people Arthur touches but ultimately that does nothing). Of course he is going to be distrustful of other people and possessive over Arthur.
Arthur feels like suddenly he can't rely on this very reliable thing that has been keeping him alive this whole time. It would shake anyone up. Especially with the shit they have to deal with? If he can't rely on directions he will be sliced to ribbons or shot without a chance of fighting back. They may as well be dead already. And for all of John's lack of agency, it's Arthur who has to actually deal with the consequences. He's the one who's getting wounded and torn to bits and has to fight all that. Plus it's HIS body. If someone's existence relies on your body, you have the right to refuse, bodily autonomy is a human right.
The problem is they've gone through this cycle before (cycle of learning and forgetting and learning that they are One Unit and need to work Together vs The Problems and forgetting) but never has it shaken Arthur so much. Even when they've argued before he would still listen to John's directions. But now? Everything is in question (as is probably the intent behind John's "failing" abilities.) They DO need other people to help and they DO need to be careful and include John. They cannot truly trust someone else while keeping John a secret or without his input. Arthur has the most control over the situation and the onus is on him to make the compromises. Yeah it's his body but 1) countless times he promised to John that they were in this together and 2) at this point separating them would be so traumatic that they may as well be One.
All of this is to say, curious that Arthur is so willing to tell Oscar about literally everything EXCEPT John. What happens if he does? What if there's someone else to assert for John in a way Arthur can't ignore so easily? Would that legitimize the shared ownership of Arthur's body? What does it mean for John to gain agency by the loss of Arthur's autonomy?
