FFXV 24-hour shipments+Digital = 5m.
Current LTD = 8.4m(PC/PS4/XB1).
I don't think 10m is realistic for a single player JRPG if FFXV is struggling to reach that sales number. This isn't the same as MHW situation where the potential of sales was held back by the hardware and region, multiplayer focused games tend to do better in the west.
KH3 isn't going to outsell FFXV and nowhere near MHW numbers.
Depends a lot on word of mouth. FFXV alienated a lot of its fan base by only focusing on the boy band main characters. I don't see KH3 making that same mistake