It wasn’t the “happy ending” people thought, i.e. he changes to a human forever. She had said in previous scenes she just wanted to see him one more time as a human. So he appeared one more time and that was the ending. So people thought he stayed as a human boy forever (No!)
But throughout the series we were told, with even a physical example (the old cat/lady who aged but her true love didn’t) that the more he changed to a human, the longer he had to stay as a cat and the shorter his whole lifespan. In the end it was two years before he could be a boy again. I think the drama ended midpoint so fans could have their happy ending (before tragedy struck, which could be the 2nd volume).
Poor Seo JH, he still waited for her after all that time. She told him she’d rather have the cat…but he gave her flowers, was always there when she needed him. She’s an idiot. But perhaps after a few more years she’ll come around. Or he’s an idiot, poor guy. The next web novel should be his story.
This is all my opinion, my musings, my fanfic outline, what I saw.