I know this is a big title for some(KH3). I could never get into it, just too old i guess.

Some in this thread are really underestimating KH3. I don't think it needs to get ported to PC and Switch to outsell GOW, but if it does get ported, it will be an easy win

You can note that I talked about "mainstream hype". I know KH is very good and has a faithful fanbase hyped about it. Of course if you look for it you will find explanations for the story and it will become a lot easier.

I'm talking about the general audience, who didn't buy huge amounts of the previous games already. I don't remember lots of IPs growing a lot all of a sudden with a new installment continuing an existing story. GoW is growing thanks to new gameplay and new storyline, but that's it. Uncharted 4 did better than the previous ones for various reasons (TLOU, new console, first huge 1st party on the console...). If the story is unrelated like the FF games or GTA, sure, no problem. 

GoW could get close to 10M sales, at least 8M is easily doable. KH went from 6+M to 4+M between the first and the second one. And it was on the PS2 userbase... I don't get why the fan base would double this time just because "we can check the journal in-game". Broad audience doesn't care about that. The question is "Can the trailers and marketing push new players to jump in this story on the way?". 

If you want to convince new players who didn't care about the IP until now, you need a story able to engage neophytes, trailers showing a very interesting gameplay with a big marketing, and awesome reviews (like 90+ on MC, the 85 zone being full of "maybe one day" games). For now, from an outsider point of view, KH is just another good JRPG with a specific audience, not made for everyone. And so it won't sell to everyone, like a lot of Japanese games releasing every year. 

I'm sure we're seeing different things. But this topic is about how outsiders like me will see the game, and how many of them will decide to try it for the first time. Because even if all the people who bought the previous games buy this one too, it will surely lose to GoW. So you can disagree with what I'm feeling and you could even be right in theory, but marketing and sales are not about theory or even truth.

I'm a fan of VN and niche Japanese games and they're awesome for me, but I get why the mainstream audience doesn't care about it. Maybe they're ignorant (and maybe I am), but at the end nobody cares about that. The game sells or not. That's it. 

No way, God of War has a bigger mainstream appeal due to its cinematicness and open world-ish nature + its protagonist has a design that appeals more to the kids and dude bro crowds. In fact, I can see kids saying: "This Kingdom Farts game is for kids, God of War is more badass",

What the heck is with this GOD frenzy? There's been tons of games, even PS exclusives, that have sold better throughout history. It's really something that I just don't understand.

This is a pretty shallow way to looks at things. 

The general public is shallow

80 million Ps4s. I doubt we can really characteristic the majority of them like that. Also, Disney is as big as ever now. We can't forget that.  

Prepare to be proven wrong if this is how you really feel

I would like KH to do well, but I suspect it wont even get close to God of War sales.

KHs has sold ~24m units across Collections/Games+Sequels&Prequels/mobile&phone (all in all like 16 releases of KHs)
God of war is probably gonna do like ~15m lifetime sales, on just the PS4 alone... theres no way KH3 beats that (on just PS4).