As a fan and a hater of NTR (Yes I fucking loath NTR but at the same time I also fucking love it idk what's wrong with me)
Yes there are Happy Ending NTRs
For example MC's wife gets pregnant and is very happy thinking that it's his kid but it is actually his old man's kid. But before that MC's father is your company Boss then MC gets promoted suddenly at work without knowing that it is because his wife is letting MC's father fuck her in exchange to the promotion. Because his wife knows that MC will be very happy and she wants him to be happy.
So it is indeed a Happy Ending to the main character.
But,
Not to us.
Hahahaha I'm just messing around I'm fking corrupted I hate myself for this.
Anyway you have a good day/night. I'm just happy that I'm able to talk to someone~