TONS of people have been asking for it, you just haven't seen it. A KH Rhythm game has been one of the most heavily requested (and actually feasible) spinoff titles since SE started making Theatrhythm games.
This. People didn't enjoy Atlantica in KH2 because of how cheesy it was, even though it was optional. But a rhythm game in itself? Oh yeah, this has been requested for YEARS. If people think Kingdom Hearts and rhythm games don't mix, they should look at the Persona franchise. Nothing about those games would make you think a dancing rhythm game would work, and yet... They're massively popular.
Kingdom Hearts is a Disney series, and Disney is known for its musical numbers, so it just made sense. I, personally, was hoping it was going to be like P4D, P3 DiM and P5DiS where you have the cast dancing to remixed versions of the KH songs we all know and love. Missed opportunity, Square, cause Ansem Seeker of Darkness seems like he'd know how to tango, and Xemnas looks like he just KNOWS ballroom dancing like the back of his hand. The meme potential would've been through the charts.
But I think the only real reason why people are disgruntled by this whole thing is because they're thinking that THIS is the big Kairi game we should be expecting, and I honestly doubt that's going to be the case. I hope not, and I think Nomura knows that people don't want this to be a one-and-done. Plus they've already put work into making her a playable character, so it'd be a waste to only use it once.
We don't even know if Melody of Memory is a 2.8 kind of deal where it's all on one Blu-Ray but Kairi's scenario and the rhythm game stuff are separate from each other. And, there's another mode we don't know about. They said "Rhythm-Action", so maybe there'll be traditional combat in it as well. There's so much to this game that we don't know about, so I think people should hold off before writing it off just yet. Plus, just like every KH game, I'm sure there'll be difficulty choices for those who aren't good at rhythm games.
If some of y'all can beat the KH3 Data Battles and Yozora, I'm sure you can handle being on beat lmao.