They had a perfectly reasonable & understandable explanation for announcing KHIII so early. 1) it had been 7,5 years since KHII, they just wanted fans to know that they are FINALLY working on THE third mainline KH & conclusion to the KH story so far and 2) they needed to show that they are going to have some serious support for next-gen consoles/PS4, just to hype the consoles a bit more & to give even early adopters a look at what the platforms would hold for them past the first year or two of the consoles' lives, instead of just the rushed launch crap.
And I don't really see what's so horrible in having official confirmation that a game exists & is being actively worked on vs. us KNOWING that some developers like Naughty Dog & Bethesda are developing Uncharted 4 & Fallout 4, but publishers refusing to officially announce them. I much prefer having that official confirmation and at least having the chance of hearing some news & seeing footage of a game, rather than getting not-so-subtle hints that basically confirm it, but not getting any actual information about who are working on it, how development is going, what we can expect of the game etc. because, hey, it hasn't been OFFICIALLY announced yet! Just sounds so drama queen-ish to whine about such a thing.
Besides, anyone who had followed Nomura's interviews/SQEX around 2012/2013 already knew Osaka team had moved on to the development of KHIII. It really didn't change much that they announced it, other than making Sony's E3 2013 press event even more HYPE. Long before KHIII even entered development it was confirmed that Dream Drop Distance would be the last major KH release before they'd move to develop KHIII, DDD itself was clearly & purely setup for the finale in KHIII and as I pointed out Nomura himself confirmed that the Osaka Team was already working on a major project soon after DDD's development ended, so they didn't really do much else but confirm what many of us had already figured out.
Just sounds so childish to whine about an early announcement like this, especially when they were relatively open about the game's state around the announcement and have remained so afterwards. It would be different if they had made it seem as if KHIII was going to be released within 12 months after E3 2013 or something. And I somewhat understand it in extreme cases (like ASOIAF, Beyond Good & Evil 2 and FFVsXIII) AFTER people have waited for years without anything to show for it & uncertainty still shrouding the projects. But by all means, they were honest about the fact that the development was still early so they didn't set any false expectations and KHIII's development seems to have advanced quite smoothly apart from the engine switch.
We have seen some pretty good & relatively polished looking direct feed gameplay from the game that shows clear progress from early builds/prototypes to a more finished product, have seen some of the new major gameplay features, know of 4 or 5 of the worlds that will be in the game etc. It's far from how VsXIII was pushed aside for over half a decade after its announcement. KHIII had been worked on for almost a year at the time of the announcement and they've continued working on it actively for these 3 years after its announcement.
There's really nothing wrong with announcing a game early. More of a problem with you obsessing about a game or being too impatient and all GIMME EVERYTHING NOW NOW NOW NOW if it's that horrible to wait for a game for a few years than there being anything wrong with publishers announcing long term plans. Movie studios like Disney/Marvel announce their movie plans for, like, 4-5 years into the future and I don't see movie folk whining about that (other than some people crying about there being so many superhero movies, creative bankruptcy etc.). Bethesda's Fallout 4 is one way to do it, but that doesn't mean everyone should wait until a few months before release to announce their games. Bethesda & Rockstar only do it because they know they have a tight enough grip of the masses balls that they don't need much hype until they are close to release. Doesn't mean that's some kind of "optimal" way of doing things.