If it turns out to be bad, then it turns out to be bad.
The Kingdom Hearts series has changed (for better or for worse) since 2005. What people long for has not been around for over a decade so if the game was to be disappointing then the writing has been on the wall for a while.
But at the same time, like Alpha Baymax said, entertainment is subjective. People do hate things that others love. For example, I find the joy in Final Fantasy XV. I loved getting out of the Regalia and exploring a path which lead me to a dungeon, all without prompts to do so. I loved climbing a volcano, which had a bunch of mystery and excitement all through it. I bonded with those boys on their trip, and when they suffered I suffered. But this is a video game that people absolutely despise. There is someone on ResetEra, who unfortunately named themselves after the great TV show The Office, who has spent the last two years trashing the game because they can't let it go (each time they make a new thread on it they promise it's the last time). They are valid to feel that way, and I'm valid to feel the way I do. And it'll be the same with Kingdom Hearts III.
Yes, there are a lot of signs to worry. Big Hero 6's cutscenes weren't complete until sometime between June - August this year, and English recording for those scenes weren't finished in time for the TGS trailer. Does this mean that world may be rushed and a bit of a mess? Sure, there's the chance and it may drag down people's enjoyment of it. Is there a chance that the world's are smaller than advertised? Of course, there has always been a chance for this to happen but back then all we could rely on was what we were being told.
People have put a lot of pressure onto Kingdom Heart's III and expect it to be the messiah of video games, something that is above every single other video game and it sends a message out to all other games that they can never compete with it. At the same time, there are people expecting a horribly short, streamlined game that meanders about with no real purpose. But none of us have touched the game at all so there's no real point to worrying about this sort of thing until we can actually go hands on.
I've said for months that people should check their expectations for the games. Bring yourselves down to a more grounded level and let's just try to enjoy what Osaka has provided. This is their first HD game for the Kingdom Hearts series, and probably for PS4 and Xbox One too. This is the first time in over a decade that the series has been pushing the boundaries of video gaming and including what other games have done for many years as well, so there's a learning curve that Osaka is probably learning from this game.
This is a game with ambitious properties. This game has such an increase in graphic fidelity, things on screen (NPCs, foliage, interactive environment), and action combat. I'm not saying that Osaka should get a free pass from the haters for this but they have put a lot of blood, sweat, and tears into this game and I hope we can all find parts of the game to enjoy.
If it's a bad game, then it's a bad game. The story could be in shambles. The combat could be a mess. The voice work could be the worst in the series. This is all possible, but because it's possible it doesn't make it so. No one on this forum has experienced the game but when we do, and if people don't like the game, I hope we can all be civilised with our reactions and responses because this has been a lot of firsts for the dev team and they can only go up from here.
And lastly. It's a fucking video game. It's not the end of the fucking world if it's a game you (general you) don't enjoy.
