I see where you're coming from, but i'll be damned if I went through all this, tried to get away from it, and then the object of my long winded affection who decided to hold me back for 2 months more on false hope realized they were just possessive of me and still didnt want me romantically.
plus the reason why an author wouldn't do this is because it'd upset a good chunk of readers and disrupt income.
im all for realistic parting or maintaining friendship despite unrequited love, but at this point in time if it came in this arc, I'd be pissed and fed up on his behalf.
This you said it perfectly. I couldn’t agree more. Even if Joo-an suddenly realized after 12 years with Seojoon that he now has romantic feelings for him, it would not be believable. Maybe if there were flashback scenes of Joo-an seeing Seojoon more than just a friend throughout the years but the fact that it’s never crossed his mind feels like he is being forced to like Seojoon, and that’s not fair for either of them. Seojoon deserves the world, and someone that does not need 12 years and a wake-up call just to love him the way he wants

The potential for the author to do something shocking, keep them as friends.
I think it would be so cool if this feeling was the fear and pain of losing someone he holds dear platonically, because that would still prove Yeojoon wrong, he still wouldn't be just a tool. It would be awesome if we could watch 2 side characters grow as people with blondie finally recognizing his feelings and Yeojoon being able to slowly recover afterwards.
But majority of romance authors just follow the typical formula and most audiences of this genre can't handle 2 attractive single people staying as friends so I know it won't happen, but it would be so cool if the writer surprised us all. ( ̄へ ̄)