Bro...of course taku stans are gonna be mad, some because he didnt end up with but its mostly because he did not get a good farewell, yes he's happy but it was still unsatisfying, the cliffhanger wasn't necessary if it was just gonna be his mother.
We're allowed to have an option & be disappointed, tho I dont agree with the aggressive comments.
Also what they said was basically "a healthy relationship wouldn't last" they're talking about it because they wouldn't get into fights or disagreements which would be bad for a relationship in someway, basically, which is another reason some taku fans are upset about

I’m sooo happy with how this one ended. I was team joowon since ch 1 and I loved this story so much. The ending was clear and the author even gave insight on how everyone would be after. Everyone hating on this story is just an upset taku stan. Like get over it, the author gave him a happy ending. They even said that had haesoo chosen taku, they wouldve eventually broken up down the line. Great story 10/10