Long story short, KH3 is for the fans of the series, both new and old alike. If you were one of the people who only played the numbered games (Aka KH1 and KH2) then it probably isn't for you. But if you have followed the series and played the games, then this is what you have been waiting for.
Also, don't let that "running with Donald and Goofy in the world of Frozen." make you think any less of the series. The disney property worlds are used for a plot device at best and a change of scenery at worst. The disney world stories are filler which doesn't contribute to the darker and more serious plot which has spanned across all the previous games. Sure, Donald Duck, Goofy and Mickey Mouse is running around, but they are not running around dealing with slap stick, but does a pretty good job at tension relief when not in "serious" mode.
Last but not least, most adults who decide to buy the game will do it for a few reasons:
1) They have been following the story and want to see the conclusion of this 16 year long story arc.
2) They are playing the games not for the story but for the fast hack n slash gameplay (or a mix of both point 1 and 2)
3) They have heard the praises of previous games (even 0.2 which is best described as a test demo for KH3 has been reported as the perfect demonstration of how 3 is going to be played) and decided to see for themselves, or just the notion of Donald Duck and Mickey Mouse running around killing dark monsters in a serious plot sounds so strange that they just HAVE to see it for themselves