I don't think it's fair that Mako is still in love with Korra and according to Bryan will always carry a touch for her but Korra has moved on to Asami whom she barley talked too in Book 4. Heck I think Korra still feels the about Mako same judging from the bedroom eyes she gave Mako in Reunion and the loving look she gave him in the Last Stand if nothing else they gave Asami so few lines with Korra compared to Mako I have to wonder if the they live happily ever line was meant to be serous
It’s one of the few things where one can legitimately say that it’s not fair to any of the characters involved.
It’s not fair to Mako because he gets to live the rest of his life knowing no one he falls for will ever really match up.
It’s not fair to Korra because instead of ending up with someone who she’s enthusiastically in love with and expresses her desire to be with, she gets to have her relationship style stripped away completely to hint at a relationship with someone she’s never demonstrated any interest in.
And it’s not fair to Asami because her father just died and all the narrative really cares about is using that to facilitate a relationship between her and Korra.
As for the “happily ever after” line, who knows? I mean, Bryke did say Korra and Mako were made for each other back in Book 1′s commentary. -_-