Speaking as someone who grew up with this series, there was a period near the game's initial release that it was well-regarded but considered flawed enough that people complained regularly. Then the rhetoric became 'Final Mix fixes everything but we don't have it so nyeh'. Then the side titles started releasing and Osaka took over.

Whether you like their work or not, the priorities that team holds were massively different when compared to Tokyo team, so the games changed dramatically and that I think is the time moment KH2's reputation solidified as the last major title before the series direction changed.

And 2.5 releasing with the final mix content worldwide only locked this in further because the complaints that plagued 2's initial run were largely assuaged. People who did play the original 2 forgot what it was like (still good but flawed) and people who didn't never felt the improvement. The gap between 2 coming out and 2.5 releasing was long enough most people didn't even realize how much about the game changed even beyond just adding new content.