I'm starting to think the root of all this disappointment and "entitlement" for KH3 is because of lack of fulfilling storywriting from the side games, or them being such a complicated convoluted mess mythos wise, that it doesn't feel as if it's adding more depth or strength to the series, but heavy chains it has to bear now. Opposite from being "Simple and Clean".
If the side games had solid stories or character arcs (or less added clones and characters), they would feel more satisfying even without the announcement of KH3, and it would be enough to tide fans over until KH3.
DDD was close to this with finally having Sora and Riku again, and they had some actually development and were given new direction (well on Riku's character anyway); along with the introduction of the Sleeping Realm, which felt like the series was getting back into the similar mythos of KH1, but it turned into a mess with the time travel thing.
BBS at least tried, but it had some weak spots wit the main characters as others have mentioned (I think was okay for the most part, but it having a Roxas clone kind of screwed with it).
Tbh I've personally been waiting more for the "period of Riku, Aqua's, and Mickey's absence" games than KH3.
Also in general I think the fans saying "they've been waiting 10 years for KH3" come from a different era in this series. They were around before any of the other side games (besides CoM) were made or even thought of; when KH1 and KH2 were about 5 years apart or less. All they had to go on for a future main title game was Mickey's letter in the secret ending, which wasn't revealed until the very end of Coded.
That, and they are probably most of all unsatisfied with the direction the series has taken since KH2, or even KH1. The bitterness comes from a place of feeling betrayed, and so feel "entitled" for KH3 to come and pay for its' sins.