XII has never been listed as one of the top games in the series. It hasn't been "vindicated by history", it's just that
XII's fanbase has coalesced into a single group, while those who hate it have moved on to other, better games. There is still a huge majority of gamers and
Final Fantasy fans who will dismiss
XII out of hand if you mention it. The discussion on it is largely positive because only people who like the game bother to discuss it anymore.
Final Fantasy VI and
VII still get haters because they are games that are largely considered fantastic by the majority of series fans (or RPG fans in general).
XII has always been considered mediocre, so it is not a game that needs to be torn off a pedestal in the minds of those who hate it. Most people who dislike the game just moved on and forgot about it.
Also, while Lightning may not be nearly as huge over here as she is in Japan, there are still tons of gamers who list her as a favorite
Final Fantasy character.
FFXIII failed over here far more because of the linearity and nonsensical plot than it did the characters. A
lot of the characters were incredibly popular. Snow, Sazh, Lightning, and Fang are all listed among a lot of people's favorite characters. Maybe not among the core
Final Fantasy fans, but among a lot of gamers in general? Definitely so.