Spoilers ahead. Please don't read if you are not looking forward to a couple of spoilers.
Twenty-Five Twenty-One is one of the most wonderful dramas I have seen. The way the Love between the main characters bloomed is really beautiful. From being each other's inspiration to becoming each other's hope and growing together to their best was well portrayed. I know the ending is not something we all rooted for however there were obvious clues at the start to let us know that this is not going to be a happy ending. Na hee do's Daughter didn't even know who Beck Yi Jin is, that definitely says something isn't t?
Coming to the break-up, yes they should not have broken up like that, however, the last scene did give us the most beautiful break-up is what I personally think. Now for those who think NHD left BYJ just because he moved to New York, I really don't think that is the case. They broke up because they were growing apart and neither of them could do anything to get it back on track. they both were busy and fully engaged in different situations that life threw at them. While BYJ was constantly sorry for not being with NHD, NHD was constantly disappointed when he didn't show up. NHD clearly says everything was not fine even before BYJ went to New york. It's just that the situations in New York that BKY had to face, just made things worse. And even though he knew that he decided to go back to New york.
Additionally even after these many years have passed, NHD didn't regret breaking up. The only thing she regrets is doing it the way she did it. In the last scene when she goes back, they are still breaking up however in a beautiful way.
Now last but not least people who are saying the writers never show where NHD's husband or BYJ's current life are, I think it's just not relevant to their story. It doesn't matter whom they married, they were each other's first love and they loved each other wholeheartedly. Again NHD didn't broke-up with BYJ because he is not near, but because she is not able to support him and she is not feeling supported by him. And once they were each other's hope and support, it's hard to be not that and just lovers.
The only loose end I felt the writer must have covered is How Ko yo rim retired and came back. However, I definitely cant say it's a flaw as the writer at least showed what happened to the couple.
the message this drama gave me is
1) Live your youth to the fullest despite knowing it won't last forever.
2) people we meet in our life won't stay forever because every person has a certain purpose in our life and they'll leave once it's over.
3) As much as we love them and embrace them when the time comes we should let them go wholeheartedly. It's better to let go and be sad for a few days than to hurt each other for a longer time.