I see people bring this up a lot, but to be honest, there are three waits for KH3. The 'soft' wait begins when you finish KH2, with Mickey's letter clearly teasing some grand new adventure. Yes, other games came out in the interim, but none of them were a large budget adventure with numbered title-ass mechanics and, most importantly, Sora, Donald and Goofy going to new Disney worlds. I loved KH2 as a kid so naturally I immediately began to wish for a follow-up, and imagined it appearing on the PS3.
The next wait came much later, when DDD's secret ending message was "This leads to KINGDOM HEARTS III". I don't think that requires too much explanation, but even without that message, DDD was clearly the game meant to establish KH3's premise and set up the final conflict.
Then, at last, came the 'true' wait, when KH3 was announced. That in and of itself was a hefty little wait, wasn't it?? 2013-2019 is no small amount of time. I was a busy little bee in college, and I STILL felt like I was going nuts waiting for that game.
But, overall, I don't really think it's THAT ridiculous to feel like you were 'waiting for KH3' since 2006, nor is it that crazy to have expected the game to drop on KH3. Obviously, things shook out very differently than most expected, and expectations don't warrant petulant, obnoxious reactions, but when we FINALLY got to SDG's new adventure in 2019, I sure as hell felt like I had waited a LONG time.