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
Jooan never said "Give me time to love you," he said, "Would losing a tool hurt this much," he's not giving him false hope. He wants time to understand his feelings and to prove to the uke that he doesnt see him as a tool. Jooan cares for him, a lot, and the uke doesnt see that. (This is where it'd be nice for them to stay as friends and have some space from each other for a while :D)
Their whole problem stems from miscommunication and assumptions from the uke assuming that Jooan saw him as nothing more than a tool and expecting Jooan to realise that he had feelings for him over the 12 years they've been together. Jooan is also at fault here. He was too wrapped up in his own ego to not see how he was treating the uke. They just need to have a serious, adult conversation and heal.

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 ̄)