it’s frustrating that they never go anywhere with mary reading john’s journal in the show, because I think it would be the catalyst to a comprehensive re-evaluation of john’s role in her sons’ lives, and is probably the only thing that would get dean to properly question his relationship to his father.
like there’s a dual refusal to accept reality going on with mary and dean - mary doesn’t want to accept that her children are grown adults and that her absence destroyed their lives. it makes her feel powerless and incredibly guilty. and dean is using mary to displace all of his unexamined rage at his father. it’s a lot easier to be angry at his mom for dying than it is to get angry at his father for abusing and neglecting them for years, especially when mary seems hellbent on retreating into fantasy where her children are still babies. but mary reading john’s journal would undo both of those things - she would be confronted with the man her husband was and the things he did to their children as a result of her absence, and she would be able to recuperate some sense of agency in her own life by getting angry at this. she can’t undo the past and she can’t fix what happened, but she can be a zealous advocate for her children and rage against what their father did to them.
and dean would have no choice but to listen, because the only person he would hear any criticism of john from is his mom. she is an authority figure he respects and wants to listen to, and on top of that she’s on his side. like for the first time in his life he would feel fully empowered to admit that he was mistreated. because his mom is telling him that it’s okay to feel that. this isn’t even something he could let himself do with bobby, I think partially because bobby isn’t biological family and partially because mary feels like a much closer authority on the subject of their father, despite the fact that bobby actually knew him a lot better.
and hilariously I think this is how sam and mary would connect and form a relationship. like she wouldn’t be able to just walk into the room one day and say well I read john’s journal and your dad was an abusive asshole. if I’d had been there I wouldn’t have let any of that happen. because dean WOULD push back and it would probably cause a fight. a serious conversation about john would have to be a slow, uneven process of untangling what the fuck happened in their childhood, and sam would happily entertain his mom asking him questions. like she can’t get straight answers from dean because he wants to both protect his mom from the reality of their life AND he wants to protect his father from the wrath of his mother. but sam would absolutely have a frank and honest conversation with mary about their childhood (at least what parts he remembers). It would be painful and messy and enraging, but he would finally have someone who wants him to talk about what happened to him and his brother. and like I’m sure mary would resist some of this at first, too, because the reality of what john did is horrifying and accepting that means accepting some kind of responsibility for it, even if she didn’t actually do any of the abusing and would have prevented it had she seen it occurring.
but like eventually all three of them would arrive at a shared set of facts, which is that john was neglectful and abusive to his kids. dean would be given permission to be angry about it from someone he loves and respects, and he would even be allowed to give up responsibility for his own anger. mary started it, so he doesn’t have to drown himself in guilt and self-doubt over thinking his father was a shitty person. he’s being handed down this mercy by his mother. and sam would finally have someone fully on his side, allowing him to present his case and explain himself. he would be given absolution for all the fights he started with his father, and for the deep-seated self-disgust he felt for being “unclean”, not to mention john’s own disgust. and mary would finally be able to feel like she knows what’s going on, and that she can take control of some element of her family’s life. she can redeem herself to her own sons by being their champion, even if it’s thirty years too late. they can do this autopsy of their own family history together and move forward with the knowledge that what happened wasn’t okay.
