It's not about "Frozen fans". It's about "Frozen fans who are invested enough to buy Kingdom Hearts III and make sure to have the hardware to play it on".Frozen is one of the highest grossing films of all time in Japan. Anyone who thinks the only Frozen fans are some specific "obsessed" demographic seems delusional. People are going to have a lot of fun with the Frozen world simply because they recognize the characters, the events, and the tunes.
BUT, maybe now is the time to say, Kingdom Hearts has never been a huge super franchise in Japan and I see no reason why that will suddenly change. The series in general is about on par with Metal Gear Solid in terms of popularity, which means KH3 will easily do over 500k, but one million is more of a ceiling than a goal.
And that's quite a difference, because if those who are most invested in Frozen are the least likely to buy KH3, which in itself has a barrier of entry, and either have or buy a PS4, then we're talking about a good ol' audience mismatch.
As mentioned in one of the posts above, series like Resident Evil and Metal Gear Solid (not to forget Final Fantasy) were huge a decade ago, but have declined a lot on PS4. And those games don't even have to deal with audience mismatches.
While it may be a bit higher than 400k in Japan, maybe 450k-500k, I've become pretty confident in my 400k/4M prediction.