Why would it ruin supernatural? Like- dean is so bisexual for castiel, And castiel is very gay for dean, Honestly it wouldn't ruin the show in any way. I think they'd be a great couple, Honestly, I hope them being canon would be added in the show, It'd be amazing
If they get together that wouldn't ruin the show, The show doesn't need to focus on their relationship for the whole time. It could be sweet things like short or long lingering kisses or hand holding or 'be careful'. It would only make sense, Otherwise why include so many scenes where Dean and Castiel are simply looking at eachother? Why include so many lines hinting at something more only to slam that door right in our faces? Castiel and Dean are meant to be, And have been in love for years.
They are clearly in love with each other and nothing would change, Except everyone knowing that they love in each other. Making it canon doesn't mean that the show has to be just about them. Also the show stopped to be about two brothers many seasons ago. It wouldnt ruin the show, But make it even better.