Surprised that so many are confidently saying FFXV. Kingdom Hearts has a huge fanbase, far more than the fanbase for any individual Final Fantasy game.
The games have also all performed well while Final Fantasy games vary greatly in sales.
Not to mention that more and more people are going off Final Fantasy all the time.
Also the 2 trailers can out the same time but KH3 got far more views.
It's also got vastly more google interest.
I don't think that KH has a bigger fanbase than any individual Final Fantasy game. FF7 for instance has a huge fanbase, it sold almost twice the copies of KH1, which is the best selling Kingdom Hearts game.
Final Fantasy sales do vary, but so far most of them have outsold KH1. Even 13, which many consider to be the worst Final Fantasy, outsold KH1.
Final Fantasy has lost fans in recent years, but that's due to diminishing quality over the course of the last few games. If anybody can make a quality Final Fantasy release again, it's Tetsuya Nomura, if it gets good reviews most of those lost fans will come back.
Not really a fair comparison, KH 3 was a new announcement, whereas XV was announced back in 2006 as Versus 13, many people skipped watching XV's trailer simply because they assumed it was more of the same footage they'd already seen in previous Versus 13 trailers.