While I agree that KHIII had many of the same problems XV had (some in a different shape or form), the final parts of the game with all the boss encounters and everything compensated the drawbacks to me personally, while FFXV's endgame felt completely undercooked.
Also, KHIII's dlcs, unlike the Royal Pack, do not change the final locations of the game or expand the core game itself - it will remain the same (for better or worse), while all this new stuff is extra scenarios. And we all knew this would happen - Final Mix was a thing for almost all the major KH games.
The KH3 DLC is the actual ending of KH3 because KH3 base game did not have one, whatever content is in ReMind would now be the "final location" of the game, even if its technically reusing all the assets from the base game for those said areas, but with different story happening in it now that is after that. The difference between KH3 DLC and XV DLC is that XV DLC are paralell sidestories to things you were doing from Noctis's perspective in the main game, to completely different things with other party members, they are not needed to understand the main game and hold no real relevance outside their DLC, while KH3's DLC is filling crucial plot points that went completely unexplained that had been set up years in advance, with things happening during the climax of the game with various characters that went completely unexplained, and with it taking place as the actual ending of the game.
This DLC is essentially the final mix, which has always been a thing.
Also, KH3 was announced in 2013 and that's when a lot of people think the development has begun. On the other hand FFXV is widely believed to have entered development in 2006, and as such XV gets more scrutiny for launching incomplete.
I'll also personally make the argument that ReMind is just one DLC that's incorporated into the story rather than 5+ DLCs that are disjointed. Moreover, everything shown in KH3 trailers mostly made it into the final game, but about half the content/scenes shown for FFXV in its trailers got cut from the final game, hence people are not upset at "stuff missing." And lastly, it seems this single DLC for KH3 is adding everything the fans asked for (refer to the KH3 forum on here for the breakdown) while 3+ years after XV came out we're still missing so much content that we've been asking for.
KH3 DLC is literally selling cut content, the overwhelming majority of it being reused assets identical to base game, and things that should have been in the base game because there was so little anything good or worthwhile in base game KH3, also the KH3 DLC looks disjointed as fuck because instead of being their own self contained side stories with different characters it's now the main story continuing through DLC from different characters which itself in KH3 was disjointed when that happened between Sora, Riku and Aqua. The XV DLC can't be said to be disjointed for that reason when the point of it was always to be seperate side stories and not the main game, even more so considering XV is a Chapter based game and so choosing a DLC episode from a menu is no different in essence than choosing a chapter from the chapter select menu. In addition KH3 also does have DLC you access from a menu too in addition to things added to main game like Royal Pack did, except unlike Royal Pack the KH3 DLC stuff being added is the actual ending of the game, whereas XV always had a proper start, middle and end that finished the story since launch.
It also took a whole year to make just for this, whereas XV pumped out far higher quality DLC in 3 month periods for a whole year for multiple full DLCs.
Also Nomura adding FF characters back into KH3 is him going back completely on what he gave as his excuse for there being no FF characters in KH3, because there was "no room", and then all the KH defenders would use Nomura's words as some gospel for why FF shouldn't be in KH anymore, but now FF is in KH3 DLC and those people clinging to Nomura's words act like they always wanted FF characters and Nomura just went back on his precious "vision", just to appease the vast majority of people who were complaining to SE for there being no FF characters in KH3.
Not gonna lie, it's kinda funny how people are eating up what essentially amounts to a Royal Expansion for KH3 while FFXV caught a ton of flack for it. I'm personally looking forward to it myself as well but I found KH3 kinda suffered the same fate as XV where a lot of the key story was backloaded and the game kinda feels like it hits its finale right as it begins to hit its stride yet I'm not seeing the same reaction to it that I saw with FFXV. Wonder why that is.
The double standard of KH/Nomura fanboys.