I do think Mia got her happy ending. There's no way Hyun can give up the life he built after all those years. It's just impossible for him to still choose Mia when he's been accustomed to the life he's currently sharing with the other four. His love for her wasn't enough. Mia's the type of FMC to get married and live a normal life, and that's not something Hyun can give her considering his lifestyle. I'm pretty sure Mia being successful and making her dreams come true was her happy ending; the rest would be up to our imagination.
I do think Mia got her happy ending. There's no way Hyun can give up the life he built after all those years. It's just impossible for him to still choose Mia when he's been accustomed to the life he's currently sharing with the other four. His love for her wasn't enough. Mia's the type of FMC to get married and live a normal life, and that's not something Hyun can give her considering his lifestyle. I'm pretty sure Mia being successful and making her dreams come true was her happy ending; the rest would be up to our imagination.