Well, since you used an anime, I'll bring up one that I feel did it in a way that avoided the trope, and did it the way I like:
Goku and Bulma were never end game, and when Bulma did get paired with someone, it was Goku's rival.
(granted, in this scenario, it also does kind of prove your point about MC's love interest just being a prize out of nowhere, since Chichi really didn't do much before or after)
He may not be much for writing romances, but at least Toriyama avoided a crappy trope with it
Also, for a /co/ example: Danny Phantom and Sam. He had at least 2 other potential love interests from what I recall....so let's say screw it and just hook him up with the main girl because we've gotta stick to the cliches.