It wasn't ALL for NOTHING, honestly Dayeon have a great impact on Dan regardless of his past being obnoxiously stupid/obsessive. And having their closure for this, I think it was worth the 10 episodes in order for them to understand each other's feelings/perspective. PLUS, readers are able to understand how Dayeon actually serves a major purpose on Dan's overall growth as a company holder and having them discuss it maturely this time is a win for both parties. Though, I do hope that the author won't bother bringing up the fact that Dayeon liked Dan again because they already sealed it with Dan saying “no” to him as a final rejection so yeah, I guess for me at least those 10 episodes for a backstory for him wasn't all for nothing.
This is by far the worst sequel to a great story. They had us for two seasons with an awful attitude from a lovesick obsessed dude who cannot handle rejection. We had at least 10 episodes of his backstory for nothing tbh.